This manual describes how to use gcj, the GNU compiler for the Java programming language. gcj can generate both .class files and object files, and it can read both Java source code and .class files.
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As gcj is just another front end to gcc, it supports many of the same options as gcc. See Option Summary. This manual only documents the options specific to gcj.
A gcj command is like a gcc command, in that it consists of a number of options and file names. The following kinds of input file names are supported:
.java.class.zip.jar.class files, all of
which are compiled. The archive may be compressed. Files in
an archive which don't end with ‘.class’ are treated as
resource files; they are compiled into the resulting object file
as ‘core:’ URLs.
@file.java source files, but that
may change.)
Each named file is compiled, just as if it had been on the command line.
.a.so-llibnameYou can specify more than one input file on the gcj command line,
in which case they will all be compiled. If you specify a
-o FILENAME
option, all the input files will be compiled together, producing a
single output file, named FILENAME.
This is allowed even when using -S or -c,
but not when using -C or --resource.
(This is an extension beyond the what plain gcc allows.)
(If more than one input file is specified, all must currently
be .java files, though we hope to fix this.)
gcj has options to control where it looks to find files it needs. For instance, gcj might need to load a class that is referenced by the file it has been asked to compile. Like other compilers for the Java language, gcj has a notion of a class path. There are several options and environment variables which can be used to manipulate the class path. When gcj looks for a given class, it searches the class path looking for matching .class or .java file. gcj comes with a built-in class path which points at the installed libgcj.jar, a file which contains all the standard classes.
In the text below, a directory or path component can refer either to an actual directory on the filesystem, or to a .zip or .jar file, which gcj will search as if it is a directory.
-Idir-I are kept in order and prepended
to the class path constructed from all the other options. Unless
compatibility with tools like javac is important, we recommend
always using -I instead of the other options for manipulating the
class path.
--classpath=path--CLASSPATH=path--classpath.
--bootclasspath=pathjava.lang.String.
--extdirs=pathCLASSPATHThe final class path is constructed like so:
-I.
CLASSPATH environment variable is specified,
then its value is appended.
Otherwise, the current directory (".") is appended.
--bootclasspath was specified, append its value.
Otherwise, append the built-in system directory, libgcj.jar.
--extdirs was specified, append the contents of the
specified directories at the end of the class path. Otherwise, append
the contents of the built-in extdirs at $(prefix)/share/java/ext.
The classfile built by gcj for the class java.lang.Object
(and placed in libgcj.jar) contains a special zero length
attribute gnu.gcj.gcj-compiled. The compiler looks for this
attribute when loading java.lang.Object and will report an error
if it isn't found, unless it compiles to bytecode (the option
-fforce-classes-archive-check can be used to override this
behavior in this particular case.)
-fforce-classes-archive-checkgnu.gcj.gcj-compiled in java.lang.Object and
issue an error if it isn't found.
-fsource=VERSIONThe Java programming language uses Unicode throughout. In an effort to integrate well with other locales, gcj allows .java files to be written using almost any encoding. gcj knows how to convert these encodings into its internal encoding at compile time.
You can use the --encoding=NAME option to specify an
encoding (of a particular character set) to use for source files. If
this is not specified, the default encoding comes from your current
locale. If your host system has insufficient locale support, then
gcj assumes the default encoding to be the ‘UTF-8’ encoding
of Unicode.
To implement --encoding, gcj simply uses the host
platform's iconv conversion routine. This means that in practice
gcj is limited by the capabilities of the host platform.
The names allowed for the argument --encoding vary from platform
to platform (since they are not standardized anywhere). However,
gcj implements the encoding named ‘UTF-8’ internally, so if
you choose to use this for your source files you can be assured that it
will work on every host.
gcj implements several warnings. As with other generic
gcc warnings, if an option of the form -Wfoo enables a
warning, then -Wno-foo will disable it. Here we've chosen to
document the form of the warning which will have an effect – the
default being the opposite of what is listed.
-Wredundant-modifierspublic.
-Wextraneous-semicolon-Wno-out-of-date-Wno-deprecated-Wunused-Wunused.
-Wall-Wredundant-modifiers -Wextraneous-semicolon
-Wunused.
To turn a Java application into an executable program,
you need to link it with the needed libraries, just as for C or C++.
The linker by default looks for a global function named main.
Since Java does not have global functions, and a
collection of Java classes may have more than one class with a
main method, you need to let the linker know which of those
main methods it should invoke when starting the application.
You can do that in any of these ways:
main method
when you link the application, using the --main flag,
described below.
gij program,
making sure that gij can find the libraries it needs.
-lgij, which links
in the main routine from the gij command.
This allows you to select the class whose main method you
want to run when you run the application. You can also use
other gij flags, such as -D flags to set properties.
Using the -lgij library (rather than the gij program
of the previous mechanism) has some advantages: it is compatible with
static linking, and does not require configuring or installing libraries.
These gij options relate to linking an executable:
--main=CLASSNAMEmain method should be invoked when the resulting executable is
run.
-Dname[=value]--main. It defines a system
property named name with value value. If value is not
specified then it defaults to the empty string. These system properties
are initialized at the program's startup and can be retrieved at runtime
using the java.lang.System.getProperty method.
-lgijgij command.
This option is an alternative to using --main; you cannot use both.
-static-libgcjCaution: Static linking of libgcj may cause essential parts
of libgcj to be omitted. Some parts of libgcj use reflection to load
classes at runtime. Since the linker does not see these references at
link time, it can omit the referred to classes. The result is usually
(but not always) a ClassNotFoundException being thrown at
runtime. Caution must be used when using this option. For more
details see:
http://gcc.gnu.org/wiki/Statically%20linking%20libgcj
In addition to the many gcc options controlling code generation, gcj has several options specific to itself.
-C--resource resource-nameResourceBundle.getBundle.
The actual file name to be compiled this way must be specified
separately.
-ftarget=VERSION-d directory-C, this causes all generated .class files
to be put in the appropriate subdirectory of directory. By
default they will be put in subdirectories of the current working
directory.
-fno-bounds-checkArrayIndexOutOfBoundsException.
-fno-store-checkArrayStoreException.
-fjni-fjni. This option causes
gcj to generate stubs which will invoke the underlying JNI
methods.
-fno-assertassert keyword. This is for compatibility
with older versions of the language specification.
-fno-optimize-static-class-initialization-O2,
gcj will try to optimize the way calls into the runtime are made
to initialize static classes upon their first use (this optimization
isn't carried out if -C was specified.) When compiling to native
code, -fno-optimize-static-class-initialization will turn this
optimization off, regardless of the optimization level in use.
--disable-assertions[=class-or-package]=class-or-package is missing disables assertion code
generation for all classes, unless overridden by a more
specific --enable-assertions flag.
If class-or-package is a class name, only disables generating
assertion checks within the named class or its inner classes.
If class-or-package is a package name, disables generating
assertion checks within the named package or a subpackage.
By default, assertions are enabled when generating class files
or when not optimizing, and disabled when generating optimized binaries.
--enable-assertions[=class-or-package]--disable-assertions.
-findirect-dispatch-findirect-dispatch option. In this mode, the code
generated by gcj honors the binary compatibility guarantees
in the Java Language Specification, and the resulting object files do
not need to be directly linked against their dependencies. Instead,
all dependencies are looked up at runtime. This allows free mixing of
interpreted and compiled code.
Note that, at present, -findirect-dispatch can only be used
when compiling .class files. It will not work when compiling
from source. CNI also does not yet work with the binary compatibility
ABI. These restrictions will be lifted in some future release.
However, if you compile CNI code with the standard ABI, you can call
it from code built with the binary compatibility ABI.
-fbootstrap-classeslibgcj that the compiled classes
should be loaded by the bootstrap loader, not the system class loader.
By default, if you compile a class and link it into an executable, it
will be treated as if it was loaded using the system class loader.
This is convenient, as it means that things like
Class.forName() will search ‘CLASSPATH’ to find the
desired class.
-freduced-reflectionFor code that does not use reflection (i.e. the methods in the
java.lang.reflect package), -freduced-reflection
will result in proper operation with a savings in executable code size.
JNI (-fjni) and the binary compatibility ABI
(-findirect-dispatch) do not work properly without full
reflection meta-data. Because of this, it is an error to use these options
with -freduced-reflection.
Caution: If there is no reflection meta-data, code that uses
a SecurityManager may not work properly. Also calling
Class.forName() may fail if the calling method has no
reflection meta-data.
Some gcj code generations options affect the resulting ABI, and
so can only be meaningfully given when libgcj, the runtime
package, is configured. libgcj puts the appropriate options from
this group into a ‘spec’ file which is read by gcj. These
options are listed here for completeness; if you are using libgcj
then you won't want to touch these options.
-fuse-boehm-gc-fhash-synchronizationsynchronize,
wait, and notify) is pointed to by a word in each object.
With this option gcj assumes that this information is stored in a
hash table and not in the object itself.
-fuse-divide-subroutine-fcheck-referencesAs we believe it is important that the Java platform not be fragmented,
gcj and libgcj try to conform to the relevant Java
specifications. However, limited manpower and incomplete and unclear
documentation work against us. So, there are caveats to using
gcj.
This list of compatibility issues is by no means complete.
assert keyword. It does not yet support the Java 2
strictfp keyword (it recognizes the keyword but ignores it).
libgcj is largely compatible with the JDK 1.2 libraries.
However, libgcj is missing many packages, most notably
java.awt. There are also individual missing classes and methods.
We currently do not have a list showing differences between
libgcj and the Java 2 platform.
libgcj implementation of a method or class differs
from the JDK implementation. This is not always a bug. Still, if it
affects you, it probably makes sense to report it so that we can discuss
the appropriate response.
libgcj. Unfortunately, programmers often want
to use newer versions of certain packages, such as those provided by
the Apache Software Foundation's Jakarta project. This has forced us
to place the org.w3c.dom and org.xml.sax packages into
their own libraries, separate from libgcj. If you intend to
use these classes, you must link them explicitly with
-l-org-w3c-dom and -l-org-xml-sax. Future versions of
gcj may not have this restriction.
The main feature of gcj is that it can compile programs written in the Java programming language to native code. Most extensions that have been added are to facilitate this functionality.
-findirect-dispatch then add them to the system-wide
classmap.db file using gcj-dbtool, they will be automatically
loaded by the libgcj system classloader. This is the new,
preferred classname-to-library resolution mechanism. See Invoking gcj-dbtool, for more information on using the classmap database.
gnu.gcj.runtime.VMClassLoader.library_control
property, but it is deprecated and will likely be removed in some
future release. When trying to load a class gnu.pkg.SomeClass
the system classloader will first try to load the shared library
lib-gnu-pkg-SomeClass.so, if that fails to load the class then
it will try to load lib-gnu-pkg.so and finally when the class
is still not loaded it will try to load lib-gnu.so. Note that
all ‘.’s will be transformed into ‘-’s and that searching
for inner classes starts with their outermost outer class. If the
class cannot be found this way the system classloader tries to use the
libgcj bytecode interpreter to load the class from the standard
classpath. This process can be controlled to some degree via the
gnu.gcj.runtime.VMClassLoader.library_control property;
See libgcj Runtime Properties.
libgcj includes a special ‘gcjlib’ URL type. A URL of
this form is like a jar URL, and looks like
‘gcjlib:/path/to/shared/library.so!/path/to/resource’. An access
to one of these URLs causes the shared library to be dlopen()d,
and then the resource is looked for in that library. These URLs are
most useful when used in conjunction with java.net.URLClassLoader.
Note that, due to implementation limitations, currently any such URL
can be accessed by only one class loader, and libraries are never
unloaded. This means some care must be exercised to make sure that
a gcjlib URL is not accessed by more than one class loader at once.
In a future release this limitation will be lifted, and such
libraries will be mapped privately.
This is a class file examiner, similar to javap. It will print
information about a number of classes, which are specified by class name
or file name.
-c--print-constants--javapjavap format. The implementation of this
feature is very incomplete.
--classpath=path--CLASSPATH=path-Idirectory-o file--help--version-v, --verbose--print-constants.
gij is a Java bytecode interpreter included with libgcj.
gij is not available on every platform; porting it requires a
small amount of assembly programming which has not been done for all the
targets supported by gcj.
The primary argument to gij is the name of a class or, with
-jar, a jar file. Options before this argument are interpreted
by gij; remaining options are passed to the interpreted program.
If a class name is specified and this class does not have a main
method with the appropriate signature (a static void method with
a String[] as its sole argument), then gij will print an
error and exit.
If a jar file is specified then gij will use information in it to
determine which class' main method will be invoked.
gij will invoke the main method with all the remaining
command-line options.
Note that gij is not limited to interpreting code. Because
libgcj includes a class loader which can dynamically load shared
objects, it is possible to give gij the name of a class which has
been compiled and put into a shared library on the class path.
-cp path-classpath pathCLASSPATH environment variable. Note that this option is
ignored if -jar is used.
-Dname[=value]java.lang.System.getProperty
method.
-ms=number-Xms.
-mx=number-Xmx.
-noverify-X-Xargument-X by itself will cause gij to list all the
supported -X options. Currently these options are supported:
-Xmssize-Xmxsize-XsssizeUnrecognized -X options are ignored, for compatibility with
other runtimes.
-jargij should be interpreted
as the name of a jar file, not a class.
--help-?--showversion--fullversion--version-verbose-verbose:classgij also recognizes and ignores the following options, for
compatibility with existing application launch scripts:
-client, -server, -hotspot, -jrockit,
-agentlib, -agentpath, -debug, -d32,
-d64, -javaagent, -noclassgc, -verify,
and -verifyremote.
gcj-dbtool is a tool for creating and manipulating class file
mapping databases. libgcj can use these databases to find a
shared library corresponding to the bytecode representation of a
class. This functionality is useful for ahead-of-time compilation of
a program that has no knowledge of gcj.
gcj-dbtool works best if all the jar files added to it are
compiled using -findirect-dispatch.
Note that gcj-dbtool is currently available as “preview
technology”. We believe it is a reasonable way to allow
application-transparent ahead-of-time compilation, but this is an
unexplored area. We welcome your comments.
-n DBFILE [SIZE]-a DBFILE JARFILE LIB-f DBFILE JARFILE LIB[-][-0] -m DBFILE DBFILE,[DBFILE]If - or -0 are used, the list of files to read is
taken from standard input instead of the command line. For
-0, Input filenames are terminated by a null character
instead of by whitespace. Useful when arguments might contain white
space. The GNU find -print0 option produces input suitable for this
mode.
-t DBFILE-l DBFILE-p--help--version-vjv-convert [OPTION] ... [INPUTFILE [OUTPUTFILE]]
jv-convert is a utility included with libgcj which
converts a file from one encoding to another. It is similar to the Unix
iconv utility.
The encodings supported by jv-convert are platform-dependent. Currently there is no way to get a list of all supported encodings.
--encoding name--from name--to nameJavaSrc encoding; this is ASCII with ‘\u’ escapes for
non-ASCII characters.
-i file-o file--reverse--help--versiongrmic [OPTION] ... class ...
grmic is a utility included with libgcj which generates
stubs for remote objects.
Note that this program isn't yet fully compatible with the JDK grmic. Some options, such as -classpath, are recognized but currently ignored. We have left these options undocumented for now.
Long options can also be given with a GNU-style leading ‘--’. For instance, --help is accepted.
-keep-keepgenerated-v1.1-vcompat-v1.2-nocompile-verbose-d directory-help-versiongc-analyze [OPTION] ... [file]
gc-analyze prints an analysis of a GC memory dump to standard out.
The memory dumps may be created by calling
gnu.gcj.util.GCInfo.enumerate(String namePrefix) from java
code. A memory dump will be created on an out of memory condition if
gnu.gcj.util.GCInfo.setOOMDump(String namePrefix) is called
before the out of memory occurs.
Running this program will create two files: TestDump001 and TestDump001.bytes.
import gnu.gcj.util.*;
import java.util.*;
public class GCDumpTest
{
static public void main(String args[])
{
ArrayList<String> l = new ArrayList<String>(1000);
for (int i = 1; i < 1500; i++) {
l.add("This is string #" + i);
}
GCInfo.enumerate("TestDump");
}
}
The memory dump may then be displayed by running:
gc-analyze -v TestDump001
--verbose-v-p tool-prefix-d directory--help--versionThis documents CNI, the Compiled Native Interface, which is is a convenient way to write Java native methods using C++. This is a more efficient, more convenient, but less portable alternative to the standard JNI (Java Native Interface).
In terms of languages features, Java is mostly a subset of C++. Java has a few important extensions, plus a powerful standard class library, but on the whole that does not change the basic similarity. Java is a hybrid object-oriented language, with a few native types, in addition to class types. It is class-based, where a class may have static as well as per-object fields, and static as well as instance methods. Non-static methods may be virtual, and may be overloaded. Overloading is resolved at compile time by matching the actual argument types against the parameter types. Virtual methods are implemented using indirect calls through a dispatch table (virtual function table). Objects are allocated on the heap, and initialized using a constructor method. Classes are organized in a package hierarchy.
All of the listed attributes are also true of C++, though C++ has extra features (for example in C++ objects may be allocated not just on the heap, but also statically or in a local stack frame). Because gcj uses the same compiler technology as G++ (the GNU C++ compiler), it is possible to make the intersection of the two languages use the same ABI (object representation and calling conventions). The key idea in CNI is that Java objects are C++ objects, and all Java classes are C++ classes (but not the other way around). So the most important task in integrating Java and C++ is to remove gratuitous incompatibilities.
You write CNI code as a regular C++ source file. (You do have to use a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
A CNI C++ source file must have:
#include <gcj/cni.h>
and then must include one header file for each Java class it uses, e.g.:
#include <java/lang/Character.h>
#include <java/util/Date.h>
#include <java/lang/IndexOutOfBoundsException.h>
These header files are automatically generated by gcjh.
CNI provides some functions and macros to make using Java objects and
primitive types from C++ easier. In general, these CNI functions and
macros start with the Jv prefix, for example the function
JvNewObjectArray. This convention is used to avoid conflicts
with other libraries. Internal functions in CNI start with the prefix
_Jv_. You should not call these; if you find a need to, let us
know and we will try to come up with an alternate solution.
Whilst a Java class is just a C++ class that doesn't mean that you are freed from the shackles of Java, a CNI C++ class must adhere to the rules of the Java programming language.
For example: it is not possible to declare a method in a CNI class
that will take a C string (char*) as an argument, or to declare a
member variable of some non-Java datatype.
The only global names in Java are class names, and packages. A package can contain zero or more classes, and also zero or more sub-packages. Every class belongs to either an unnamed package or a package that has a hierarchical and globally unique name.
A Java package is mapped to a C++ namespace. The Java class
java.lang.String is in the package java.lang, which is a
sub-package of java. The C++ equivalent is the class
java::lang::String, which is in the namespace java::lang
which is in the namespace java.
Here is how you could express this:
(// Declare the class(es), possibly in a header file:
namespace java {
namespace lang {
class Object;
class String;
...
}
}
class java::lang::String : public java::lang::Object
{
...
};
The gcjh tool automatically generates the necessary namespace
declarations.
Always using the fully-qualified name of a java class can be
tiresomely verbose. Using the full qualified name also ties the code
to a single package making code changes necessary should the class
move from one package to another. The Java package declaration
specifies that the following class declarations are in the named
package, without having to explicitly name the full package
qualifiers. The package declaration can be
followed by zero or more import declarations, which
allows either a single class or all the classes in a package to be
named by a simple identifier. C++ provides something similar with the
using declaration and directive.
In Java:
import package-name.class-name;
allows the program text to refer to class-name as a shorthand for
the fully qualified name: package-name.class-name.
To achieve the same effect C++, you have to do this:
using package-name::class-name;
Java can also cause imports on demand, like this:
import package-name.*;
Doing this allows any class from the package package-name to be referred to only by its class-name within the program text.
The same effect can be achieved in C++ like this:
using namespace package-name;
Java provides 8 primitives types which represent integers, floats, characters and booleans (and also the void type). C++ has its own very similar concrete types. Such types in C++ however are not always implemented in the same way (an int might be 16, 32 or 64 bits for example) so CNI provides a special C++ type for each primitive Java type:
| Java type | C/C++ typename | Description
|
char | jchar | 16 bit Unicode character
|
boolean | jboolean | logical (true or false) values
|
byte | jbyte | 8-bit signed integer
|
short | jshort | 16 bit signed integer
|
int | jint | 32 bit signed integer
|
long | jlong | 64 bit signed integer
|
float | jfloat | 32 bit IEEE floating point number
|
double | jdouble | 64 bit IEEE floating point number
|
void | void | no value
|
When referring to a Java type You should always use these C++ typenames (e.g.: jint)
to avoid disappointment.
In Java each primitive type has an associated reference type,
e.g.: boolean has an associated java.lang.Boolean.TYPE class.
In order to make working with such classes easier GCJ provides the macro
JvPrimClass:
Return a pointer to the
Classobject corresponding to the type supplied.JvPrimClass(void) java.lang.Void.TYPE
A Java reference type is treated as a class in C++. Classes and
interfaces are handled this way. A Java reference is translated to a
C++ pointer, so for instance a Java java.lang.String becomes,
in C++, java::lang::String *.
CNI provides a few built-in typedefs for the most common classes:
| Java type | C++ typename | Description
|
java.lang.Object | jobject | Object type
|
java.lang.String | jstring | String type
|
java.lang.Class | jclass | Class type
|
Class
instance. These can be accessed in CNI via the static class$
field of a class. The class$ field is of type Class
(and not Class *), so you will typically take the address of
it.
Here is how you can refer to the class of String, which in
Java would be written String.class:
using namespace java::lang;
doSomething (&String::class$);
A Java class can implement zero or more interfaces, in addition to inheriting from a single base class.
CNI allows CNI code to implement methods of interfaces. You can also call methods through interface references, with some limitations.
CNI doesn't understand interface inheritance at all yet. So, you can only call an interface method when the declared type of the field being called matches the interface which declares that method. The workaround is to cast the interface reference to the right superinterface.
For example if you have:
interface A
{
void a();
}
interface B extends A
{
void b();
}
and declare a variable of type B in C++, you can't call
a() unless you cast it to an A first.
All Java classes are derived from java.lang.Object. C++ does
not have a unique root class, but we use the C++ class
java::lang::Object as the C++ version of the
java.lang.Object Java class. All other Java classes are mapped
into corresponding C++ classes derived from java::lang::Object.
Interface inheritance (the implements keyword) is currently not
reflected in the C++ mapping.
Each object contains an object header, followed by the instance fields of the class, in order. The object header consists of a single pointer to a dispatch or virtual function table. (There may be extra fields in front of the object, for example for memory management, but this is invisible to the application, and the reference to the object points to the dispatch table pointer.)
The fields are laid out in the same order, alignment, and size as in
C++. Specifically, 8-bit and 16-bit native types (byte,
short, char, and boolean) are not widened
to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
to 32 bits when on the VM stack or temporary registers.
If you include the gcjh-generated header for a
class, you can access fields of Java classes in the natural
way. For example, given the following Java class:
public class Int
{
public int i;
public Int (int i) { this.i = i; }
public static Int zero = new Int(0);
}
you can write:
#include <gcj/cni.h>;
#include <Int>;
Int*
mult (Int *p, jint k)
{
if (k == 0)
return Int::zero; // Static member access.
return new Int(p->i * k);
}
CNI does not strictly enforce the Java access specifiers, because Java permissions cannot be directly mapped into C++ permission. Private Java fields and methods are mapped to private C++ fields and methods, but other fields and methods are mapped to public fields and methods.
Java requires that each class be automatically initialized at the time
of the first active use. Initializing a class involves
initializing the static fields, running code in class initializer
methods, and initializing base classes. There may also be
some implementation specific actions, such as allocating
String objects corresponding to string literals in
the code.
The GCJ compiler inserts calls to JvInitClass at appropriate
places to ensure that a class is initialized when required. The C++
compiler does not insert these calls automatically—it is the
programmer's responsibility to make sure classes are initialized.
However, this is fairly painless because of the conventions assumed by
the Java system.
First, libgcj will make sure a class is initialized before an
instance of that object is created. This is one of the
responsibilities of the new operation. This is taken care of
both in Java code, and in C++ code. When G++ sees a new of a
Java class, it will call a routine in libgcj to allocate the
object, and that routine will take care of initializing the class.
Note however that this does not happen for Java arrays; you must
allocate those using the appropriate CNI function. It follows that
you can access an instance field, or call an instance (non-static)
method and be safe in the knowledge that the class and all of its base
classes have been initialized.
Invoking a static method is also safe. This is because the
Java compiler adds code to the start of a static method to make sure
the class is initialized. However, the C++ compiler does not
add this extra code. Hence, if you write a native static method
using CNI, you are responsible for calling JvInitClass
before doing anything else in the method (unless you are sure
it is safe to leave it out).
Accessing a static field also requires the class of the
field to be initialized. The Java compiler will generate code
to call JvInitClass before getting or setting the field.
However, the C++ compiler will not generate this extra code,
so it is your responsibility to make sure the class is
initialized before you access a static field from C++.
New Java objects are allocated using a class instance creation expression, e.g.:
new Type ( ... )
The same syntax is used in C++. The main difference is that C++ objects have to be explicitly deleted; in Java they are automatically deleted by the garbage collector. Using CNI, you can allocate a new Java object using standard C++ syntax and the C++ compiler will allocate memory from the garbage collector. If you have overloaded constructors, the compiler will choose the correct one using standard C++ overload resolution rules.
For example:
java::util::Hashtable *ht = new java::util::Hashtable(120);
When allocating memory in CNI methods it is best to handle out-of-memory conditions by throwing a Java exception. These functions are provided for that purpose:
Calls malloc. Throws
java.lang.OutOfMemoryErrorif allocation fails.
Calls realloc. Throws
java.lang.OutOfMemoryErrorif reallocation fails.
While in many ways Java is similar to C and C++, it is quite different
in its treatment of arrays. C arrays are based on the idea of pointer
arithmetic, which would be incompatible with Java's security
requirements. Java arrays are true objects (array types inherit from
java.lang.Object). An array-valued variable is one that
contains a reference (pointer) to an array object.
Referencing a Java array in C++ code is done using the
JArray template, which as defined as follows:
class __JArray : public java::lang::Object
{
public:
int length;
};
template<class T>
class JArray : public __JArray
{
T data[0];
public:
T& operator[](jint i) { return data[i]; }
};
There are a number of typedefs which correspond to typedefs
from the JNI. Each is the type of an array holding objects
of the relevant type:
typedef __JArray *jarray;
typedef JArray<jobject> *jobjectArray;
typedef JArray<jboolean> *jbooleanArray;
typedef JArray<jbyte> *jbyteArray;
typedef JArray<jchar> *jcharArray;
typedef JArray<jshort> *jshortArray;
typedef JArray<jint> *jintArray;
typedef JArray<jlong> *jlongArray;
typedef JArray<jfloat> *jfloatArray;
typedef JArray<jdouble> *jdoubleArray;
This template function can be used to get a pointer to the elements of the
array. For instance, you can fetch a pointer to the integers that make up anint[]like so:extern jintArray foo; jint *intp = elements (foo);The name of this function may change in the future.
This creates a new array whose elements have reference type.
klassis the type of elements of the array andinitis the initial value put into every slot in the array.
using namespace java::lang;
JArray<String *> *array
= (JArray<String *> *) JvNewObjectArray(length, &String::class$, NULL);
For each primitive type there is a function which can be used to create a new array of that type. The name of the function is of the form:
JvNewTypeArray
For example:
JvNewBooleanArray
can be used to create an array of Java primitive boolean types.
The following function definition is the template for all such functions:
Java methods are mapped directly into C++ methods.
The header files generated by gcjh
include the appropriate method definitions.
Basically, the generated methods have the same names and
corresponding types as the Java methods,
and are called in the natural manner.
Both Java and C++ provide method overloading, where multiple
methods in a class have the same name, and the correct one is chosen
(at compile time) depending on the argument types.
The rules for choosing the correct method are (as expected) more complicated
in C++ than in Java, but given a set of overloaded methods
generated by gcjh the C++ compiler will choose
the expected one.
Common assemblers and linkers are not aware of C++ overloading, so the standard implementation strategy is to encode the parameter types of a method into its assembly-level name. This encoding is called mangling, and the encoded name is the mangled name. The same mechanism is used to implement Java overloading. For C++/Java interoperability, it is important that both the Java and C++ compilers use the same encoding scheme.
Static Java methods are invoked in CNI using the standard
C++ syntax, using the :: operator rather
than the . operator.
For example:
jint i = java::lang::Math::round((jfloat) 2.3);
C++ method definition syntax is used to define a static native method. For example:
#include <java/lang/Integer>
java::lang::Integer*
java::lang::Integer::getInteger(jstring str)
{
...
}
Constructors are called implicitly as part of object allocation
using the new operator.
For example:
java::lang::Integer *x = new java::lang::Integer(234);
Java does not allow a constructor to be a native method. This limitation can be coded round however because a constructor can call a native method.
Calling a Java instance method from a C++ CNI method is done using the standard C++ syntax, e.g.:
// First create the Java object. java::lang::Integer *x = new java::lang::Integer(234); // Now call a method. jint prim_value = x->intValue(); if (x->longValue == 0) ...
Defining a Java native instance method is also done the natural way:
#include <java/lang/Integer.h>
jdouble
java::lang:Integer::doubleValue()
{
return (jdouble) value;
}
In Java you can call a method using an interface reference. This is supported, but not completely. See Interfaces.
CNI provides a number of utility functions for
working with Java Java String objects.
The names and interfaces are analogous to those of JNI.
Returns a Java
Stringobject with characters from the array of Unicode characters chars up to the index len in that array.
Returns a Java
Stringmade up of len bytes from bytes.
As above but the length of the
Stringisstrlen(bytes).
Returns a
Stringwhich is made up of the UTF encoded characters present in the C string bytes.
Returns a pointer to an array of characters making up the
Stringstr.
Returns the number of bytes required to encode the contents of the
Stringstr in UTF-8.
Puts the UTF-8 encoding of a region of the
Stringstr into the bufferbuf. The region to fetch is marked by start and len.Note that buf is a buffer, not a C string. It is not null terminated.
Because CNI is designed to represent Java classes and methods it cannot be mixed readily with C/C++ types.
One important restriction is that Java classes cannot have non-Java type instance or static variables and cannot have methods which take non-Java types as arguments or return non-Java types.
None of the following is possible with CNI:
class ::MyClass : public java::lang::Object
{
char* variable; // char* is not a valid Java type.
}
uint
::SomeClass::someMethod (char *arg)
{
.
.
.
} // uint is not a valid Java type, neither is char*
Of course, it is ok to use C/C++ types within the scope of a method:
jint
::SomeClass::otherMethod (jstring str)
{
char *arg = ...
.
.
.
}
The above restriction can be problematic, so CNI includes the
gnu.gcj.RawData class. The RawData class is a
non-scanned reference type. In other words variables declared
of type RawData can contain any data and are not checked by the
compiler or memory manager in any way.
This means that you can put C/C++ data structures (including classes) in your CNI classes, as long as you use the appropriate cast.
Here are some examples:
class ::MyClass : public java::lang::Object
{
gnu.gcj.RawData string;
MyClass ();
gnu.gcj.RawData getText ();
void printText ();
}
::MyClass::MyClass ()
{
char* text = ...
string = text;
}
gnu.gcj.RawData
::MyClass::getText ()
{
return string;
}
void
::MyClass::printText ()
{
printf("%s\n", (char*) string);
}
gnu.gcj.RawDataManaged is another type used to indicate special data used
by native code. Unlike the RawData type, fields declared as
RawDataManaged will be "marked" by the memory manager and
considered for garbage collection.
Native data which is allocated using CNI's JvAllocBytes()
function and stored in a RawDataManaged will be automatically
freed when the Java object it is associated with becomes unreachable.
Allocates size bytes from the heap. The memory returned is zeroed. This memory is not scanned for pointers by the garbage collector, but will be freed if no references to it are discovered.
This function can be useful if you need to associate some native data with a Java object. Using a CNI's special
RawDataManagedtype, native data allocated withJvAllocByteswill be automatically freed when the Java object itself becomes unreachable.
On Posix based systems the libgcj library uses several signals
internally. CNI code should not attempt to use the same
signals as doing so may cause libgcj and/or the CNI
code to fail.
SIGSEGV is used on many systems to generate
NullPointerExceptions. SIGCHLD is used internally by
Runtime.exec(). Several other signals (that vary from platform to
platform) can be used by the memory manager and by
Thread.interrupt().
While C++ and Java share a common exception handling framework, things are not yet perfectly integrated. The main issue is that the run-time type information facilities of the two languages are not integrated.
Still, things work fairly well. You can throw a Java exception from
C++ using the ordinary throw construct, and this
exception can be caught by Java code. Similarly, you can catch an
exception thrown from Java using the C++ catch
construct.
Here is an example:
if (i >= count)
throw new java::lang::IndexOutOfBoundsException();
Normally, G++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code:
struct S { ~S(); };
extern void bar(); // Is implemented in Java and may throw exceptions.
void foo()
{
S s;
bar();
}
The usual effect of an incorrect guess is a link failure, complaining of
a missing routine called __gxx_personality_v0.
You can inform the compiler that Java exceptions are to be used in a
translation unit, irrespective of what it might think, by writing
#pragma GCC java_exceptions at the head of the
file. This #pragma must appear before any
functions that throw or catch exceptions, or run destructors when
exceptions are thrown through them.
Each Java object has an implicit monitor.
The Java VM uses the instruction monitorenter to acquire
and lock a monitor, and monitorexit to release it.
The corresponding CNI macros are JvMonitorEnter and
JvMonitorExit (JNI has similar methods MonitorEnter
and MonitorExit).
The Java source language does not provide direct access to these primitives.
Instead, there is a synchronized statement that does an
implicit monitorenter before entry to the block,
and does a monitorexit on exit from the block.
Note that the lock has to be released even when the block is abnormally
terminated by an exception, which means there is an implicit
try finally surrounding synchronization locks.
From C++, it makes sense to use a destructor to release a lock. CNI defines the following utility class:
class JvSynchronize() {
jobject obj;
JvSynchronize(jobject o) { obj = o; JvMonitorEnter(o); }
~JvSynchronize() { JvMonitorExit(obj); }
};
So this Java code:
synchronized (OBJ)
{
CODE
}
might become this C++ code:
{
JvSynchronize dummy (OBJ);
CODE;
}
Java also has methods with the synchronized attribute.
This is equivalent to wrapping the entire method body in a
synchronized statement.
(Alternatively, an implementation could require the caller to do
the synchronization. This is not practical for a compiler, because
each virtual method call would have to test at run-time if
synchronization is needed.) Since in gcj
the synchronized attribute is handled by the
method implementation, it is up to the programmer
of a synchronized native method to handle the synchronization
(in the C++ implementation of the method).
In other words, you need to manually add JvSynchronize
in a native synchronized method.
CNI permits C++ applications to make calls into Java classes, in addition to allowing Java code to call into C++. Several functions, known as the invocation API, are provided to support this.
Initializes the Java runtime. This function performs essential initialization of the threads interface, garbage collector, exception handling and other key aspects of the runtime. It must be called once by an application with a non-Java
main()function, before any other Java or CNI calls are made. It is safe, but not recommended, to callJvCreateJavaVM()more than once provided it is only called from a single thread. The vmargs parameter can be used to specify initialization parameters for the Java runtime. It may beNULL.JvVMInitArgs represents a list of virtual machine initialization arguments.
JvCreateJavaVM()ignores the version field.typedef struct JvVMOption { // a VM initialization option char* optionString; // extra information associated with this option void* extraInfo; } JvVMOption; typedef struct JvVMInitArgs { // for compatibility with JavaVMInitArgs jint version; // number of VM initialization options jint nOptions; // an array of VM initialization options JvVMOption* options; // true if the option parser should ignore unrecognized options jboolean ignoreUnrecognized; } JvVMInitArgs;
JvCreateJavaVM()returns0upon success, or-1if the runtime is already initialized.Note: In GCJ 3.1, the
vm_argsparameter is ignored. It is recognized and used as of release 4.0.
Registers an existing thread with the Java runtime. This must be called once from each thread, before that thread makes any other Java or CNI calls. It must be called after
JvCreateJavaVM. name specifies a name for the thread. It may beNULL, in which case a name will be generated. group is the ThreadGroup in which this thread will be a member. If it isNULL, the thread will be a member of the main thread group. The return value is the JavaThreadobject that represents the thread. It is safe to callJvAttachCurrentThread()more than once from the same thread. If the thread is already attached, the call is ignored and the current thread object is returned.
Unregisters a thread from the Java runtime. This should be called by threads that were attached using
JvAttachCurrentThread(), after they have finished making calls to Java code. This ensures that any resources associated with the thread become eligible for garbage collection. This function returns0upon success, or-1if the current thread is not attached.
If an exception is thrown from Java code called using the invocation API, and no handler for the exception can be found, the runtime will abort the application. In order to make the application more robust, it is recommended that code which uses the invocation API be wrapped by a top-level try/catch block that catches all Java exceptions.
The following code demonstrates the use of the invocation API. In this
example, the C++ application initializes the Java runtime and attaches
itself. The java.lang.System class is initialized in order to
access its out field, and a Java string is printed. Finally, the thread
is detached from the runtime once it has finished making Java calls. Everything
is wrapped with a try/catch block to provide a default handler for any uncaught
exceptions.
The example can be compiled with c++ -c test.cc; gcj test.o.
// test.cc
#include <gcj/cni.h>
#include <java/lang/System.h>
#include <java/io/PrintStream.h>
#include <java/lang/Throwable.h>
int main(int argc, char *argv[])
{
using namespace java::lang;
try
{
JvCreateJavaVM(NULL);
JvAttachCurrentThread(NULL, NULL);
String *message = JvNewStringLatin1("Hello from C++");
JvInitClass(&System::class$);
System::out->println(message);
JvDetachCurrentThread();
}
catch (Throwable *t)
{
System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
t->printStackTrace();
}
}
Reflection is possible with CNI code, it functions similarly to how it functions with JNI.
The types jfieldID and jmethodID
are as in JNI.
The functions:
JvFromReflectedField,
JvFromReflectedMethod,
JvToReflectedField
JvToFromReflectedMethod
will be added shortly, as will other functions corresponding to JNI.
The runtime behavior of the libgcj library can be modified by setting
certain system properties. These properties can be compiled into the program
using the -Dname[=value] option to gcj or by
setting them explicitly in the program by calling the
java.lang.System.setProperty() method. Some system properties are only
used for informational purposes (like giving a version number or a user name).
A program can inspect the current value of a property by calling the
java.lang.System.getProperty() method.
The following properties are normally found in all implementations of the core libraries for the Java language.
java.versionlibgcj version number.
java.vendorjava.vendor.urljava.homegcj was installed. Taken from the --prefix
option given to configure.
java.class.versionjava.vm.specification.versionlibgcj.
(Currently ‘1.0’)
java.vm.specification.vendorjava.vm.specification.namejava.vm.versionjava.vm.vendorjava.vm.namejava.specification.versionlibgcj.
(Currently set to ‘1.3’)
java.specification.vendorjava.specification.namejava.class.pathjava.library.pathjava.io.tmpdirjava.compilerlibgcj.
java.ext.dirsjava.protocol.handler.pkgsjava.net.URL.
java.rmi.server.codebasejava.rmi.server.RMIClassLoader
to load classes from.
jdbc.driversjava.sql.DriverManager
when it starts up.
file.separatorfile.encodingpath.separatorline.separatorpolicy.providerjava.security.Policy.getPolicy.
user.nameuser.homeuser.diruser.languagejava.util.Locale class.
user.regionjava.util.Local class.
user.variantuser.timezonejava.util.TimeZone class.
os.nameos.archos.versionawt.appletWarningjava.awt.Window.getWarningString() when the window is
“insecure”.
awt.toolkitjava.awt.Toolkit.
Defaults to gnu.awt.gtk.GtkToolkit.
http.proxyHosthttp.proxyPortlibgcj is based on the GNU Classpath (Essential Libraries for Java) a
GNU project to create free core class libraries for use with virtual machines
and compilers for the Java language. The following properties are common to
libraries based on GNU Classpath.
gcj.dumpobjectjava.io.ObjectInput and
java.io.ObjectOutput classes when set to something else then the empty
string. Only used when running a debug build of the library.
gnu.classpath.vm.shortnamelibgcj,
this will always be ‘libgcj’.
gnu.classpath.home.urlThe following properties are specific to the libgcj runtime and will
normally not be found in other core libraries for the java language.
java.fullversionjava.vm.name and java.vm.version.
java.vm.infojava.fullversion.
impl.prefixjava.net.DatagramSocket class when set to something else
then the empty string. When set all newly created DatagramSockets will
try to load a class java.net.[impl.prefix]DatagramSocketImpl instead of
the normal java.net.PlainDatagramSocketImpl.
gnu.gcj.prognamegij front end is
used, or the program binary name in the case where an application is compiled
to a native binary.
gnu.gcj.user.realnamegnu.gcj.runtime.NameFinder.use_addr2linefalse
may suppress line numbers when printing stack traces and when using
the java.util.logging infrastructure. However, performance may improve
significantly for applications that print stack traces or make logging calls
frequently.
gnu.gcj.runtime.NameFinder.show_rawtrue will cause the name
of the object and the offset within that object to be printed when no
line number is available. This allows for off-line decoding of
stack traces if necessary debug information is available. The default
is false, no raw addresses are printed.
gnu.gcj.runtime.NameFinder.remove_unknowntrue, stack frames for non-java
code are suppressed. Setting this to false will cause any
non-java stack frames to be printed in addition to frames for the java
code.
gnu.gcj.runtime.VMClassLoader.library_controlgnu.gcj.runtime.endorsed.dirsjava.endorsed.dirs, property, but
specifies some extra directories which are searched after the standard
endorsed directories. This is primarily useful for telling
libgcj about additional libraries which are ordinarily
incorporated into the JDK, and which should be loaded by the bootstrap
class loader, but which are not yet part of libgcj itself for
some reason.
gnu.gcj.jit.compilerClassLoader.defineClass is called. If not set, gcj
will not be invoked by the runtime; this can also be controlled via
Compiler.disable.
gnu.gcj.jit.optionsgnu.gcj.jit.cachedirgnu.gcj.precompiled.db.pathlibgcj to
find shared libraries corresponding to classes that are loaded from
bytecode. libgcj often has a built-in default database; it
can be queried using gcj-dbtool -p.
While writing gcj and libgcj we have, of course, relied
heavily on documentation from Sun Microsystems. In particular we have
used The Java Language Specification (both first and second editions),
the Java Class Libraries (volumes one and two), and the Java Virtual
Machine Specification. In addition we've used the online documentation
at http://java.sun.com/.
The current gcj home page is http://gcc.gnu.org/java/.
For more information on gcc, see http://gcc.gnu.org/.
Some libgcj testing is done using the Mauve test suite. This is
a free software Java class library test suite which is being written
because the JCK is not free. See
http://sources.redhat.com/mauve/ for more information.
elements on template<class T>: ArraysJvAllocBytes: Mixing with C++JvAttachCurrentThread: InvocationJvCreateJavaVM: InvocationJvDetachCurrentThread: InvocationJvFree: Memory allocationJvGetArrayLength: ArraysJvGetStringChars: StringsJvGetStringUTFLength: StringsJvGetStringUTFRegion: StringsJvMalloc: Memory allocationJvNewBooleanArray: ArraysJvNewObjectArray: ArraysJvNewString: StringsJvNewStringLatin1: StringsJvNewStringUTF: StringsJvPrimClass: Primitive typesJvRealloc: Memory allocation