This is gcj.info, produced by makeinfo version 4.5 from gcj.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * Gcj: (gcj). Ahead-of-time compiler for the Java language END-INFO-DIR-ENTRY INFO-DIR-SECTION Individual utilities START-INFO-DIR-ENTRY * gcjh: (gcj)Invoking gcjh. Generate header files from Java class files * jv-scan: (gcj)Invoking jv-scan. Print information about Java source files * jcf-dump: (gcj)Invoking jcf-dump. Print information about Java class files * gij: (gcj)Invoking gij. GNU interpreter for Java bytecode * jv-convert: (gcj)Invoking jv-convert. Convert file from one encoding to another * rmic: (gcj)Invoking rmic. Generate stubs for Remote Method Invocation. * rmiregistry: (gcj)Invoking rmiregistry. The remote object registry. END-INFO-DIR-ENTRY Copyright (C) 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gcj.info, Node: Code Generation, Next: Configure-time Options, Prev: Warnings, Up: Invoking gcj Code Generation =============== In addition to the many `gcc' options controlling code generation, `gcj' has several options specific to itself. `--main=CLASSNAME' This option is used when linking to specify the name of the class whose `main' method should be invoked when the resulting executable is run. (1) `-DNAME[=VALUE]' This option can only be used with `--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. `-C' This option is used to tell `gcj' to generate bytecode (`.class' files) rather than object code. `--resource RESOURCE-NAME' This option is used to tell `gcj' to compile the contents of a given file to object code so it may be accessed at runtime with the core protocol handler as `core:/RESOURCE-NAME'. Note that RESOURCE-NAME is the name of the resource as found at runtime; for instance, it could be used in a call to `ResourceBundle.getBundle'. The actual file name to be compiled this way must be specified separately. `-d DIRECTORY' When used with `-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-check' By default, `gcj' generates code which checks the bounds of all array indexing operations. With this option, these checks are omitted, which can improve performance for code that uses arrays extensively. Note that this can result in unpredictable behavior if the code in question actually does violate array bounds constraints. It is safe to use this option if you are sure that your code will never throw an `ArrayIndexOutOfBoundsException'. `-fno-store-check' Don't generate array store checks. When storing objects into arrays, a runtime check is normally generated in order to ensure that the object is assignment compatible with the component type of the array (which may not be known at compile-time). With this option, these checks are omitted. This can improve performance for code which stores objects into arrays frequently. It is safe to use this option if you are sure your code will never throw an `ArrayStoreException'. `-fjni' With `gcj' there are two options for writing native methods: CNI and JNI. By default `gcj' assumes you are using CNI. If you are compiling a class with native methods, and these methods are implemented using JNI, then you must use `-fjni'. This option causes `gcj' to generate stubs which will invoke the underlying JNI methods. `-fno-optimize-static-class-initialization' When the optimization level is greather or equal to `-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. ---------- Footnotes ---------- (1) 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.  File: gcj.info, Node: Configure-time Options, Prev: Code Generation, Up: Invoking gcj Configure-time Options ====================== 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' This enables the use of the Boehm GC bitmap marking code. In particular this causes `gcj' to put an object marking descriptor into each vtable. `-fhash-synchronization' By default, synchronization data (the data used for `synchronize', `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' On some systems, a library routine is called to perform integer division. This is required to get exception handling correct when dividing by zero. `-fcheck-references' On some systems it's necessary to insert inline checks whenever accessing an object via a reference. On other systems you won't need this because null pointer accesses are caught automatically by the processor.  File: gcj.info, Node: Compatibility, Next: Invoking gcjh, Prev: Invoking gcj, Up: Top Compatibility with the Java Platform ************************************ As 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. * `gcj' implements the JDK 1.1 language. It supports inner classes, though these are known to still be buggy. 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. * Sometimes the `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.  File: gcj.info, Node: Invoking gcjh, Next: Invoking jv-scan, Prev: Compatibility, Up: Top Invoking gcjh ************* The `gcjh' program is used to generate header files from class files. It can generate both CNI and JNI header files, as well as stub implementation files which can be used as a basis for implementing the required native methods. `-stubs' This causes `gcjh' to generate stub files instead of header files. By default the stub file will be named after the class, with a suffix of `.cc'. In JNI mode, the default output file will have the suffix `.c'. `-jni' This tells `gcjh' to generate a JNI header or stub. By default, CNI headers are generated. `-add TEXT' Inserts TEXT into the class body. This is ignored in JNI mode. `-append TEXT' Inserts TEXT into the header file after the class declaration. This is ignored in JNI mode. `-friend TEXT' Inserts TEXT into the class as a `friend' declaration. This is ignored in JNI mode. `-prepend TEXT' Inserts TEXT into the header file before the class declaration. This is ignored in JNI mode. `--classpath=PATH' `--CLASSPATH=PATH' `-IDIRECTORY' `-d DIRECTORY' `-o FILE' These options are all identical to the corresponding `gcj' options. `-o FILE' Sets the output file name. This cannot be used if there is more than one class on the command line. `-td DIRECTORY' Sets the name of the directory to use for temporary files. `-M' Print all dependencies to stdout; suppress ordinary output. `-MM' Print non-system dependencies to stdout; suppress ordinary output. `-MD' Print all dependencies to stdout. `-MMD' Print non-system dependencies to stdout. `--help' Print help about `gcjh' and exit. No further processing is done. `--version' Print version information for `gcjh' and exit. No further processing is done. `-v, --verbose' Print extra information while running. All remaining options are considered to be names of classes.  File: gcj.info, Node: Invoking jv-scan, Next: Invoking jcf-dump, Prev: Invoking gcjh, Up: Top Invoking jv-scan **************** The `jv-scan' program can be used to print information about a Java source file (`.java' file). `--complexity' This prints a complexity measure, related to cyclomatic complexity, for each input file. `--encoding=NAME' This works like the corresponding `gcj' option. `--print-main' This prints the name of the class in this file containing a `main' method. `--list-class' This lists the names of all classes defined in the input files. `--list-filename' If `--list-class' is given, this option causes `jv-scan' to also print the name of the file in which each class was found. `-o FILE' Print output to the named file. `--help' Print help, then exit. `--version' Print version number, then exit.  File: gcj.info, Node: Invoking jcf-dump, Next: Invoking gij, Prev: Invoking jv-scan, Up: Top Invoking jcf-dump ***************** This is a class file examiner, similar to `javap'. It will print information about a number of classes, which are specifed by class name or file name. `-c' Disassemble method bodies. By default method bodies are not printed. `--javap' Generate output in `javap' format. The implementation of this feature is very incomplete. `--classpath=PATH' `--CLASSPATH=PATH' `-IDIRECTORY' `-o FILE' These options as the same as the corresponding `gcj' options. `--help' Print help, then exit. `--version' Print version number, then exit. `-v, --verbose' Print extra information while running.  File: gcj.info, Node: Invoking gij, Next: Invoking jv-convert, Prev: Invoking jcf-dump, Up: Top Invoking gij ************ `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. `-DNAME[=VALUE]' This 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. `-ms=NUMBER' This sets the initial heap size. `-mx=NUMBER' This sets the maximum heap size. `-jar' This indicates that the name passed to `gij' should be interpreted as the name of a jar file, not a class. `--help' Print help, then exit. `--version' Print version number, then exit.  File: gcj.info, Node: Invoking jv-convert, Next: Invoking rmic, Prev: Invoking gij, Up: Top Invoking jv-convert ******************* `jv-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' Use NAME as the input encoding. The default is the current locale's encoding. `--to NAME' Use NAME as the output encoding. The default is the `JavaSrc' encoding; this is ASCII with `\u' escapes for non-ASCII characters. `-i FILE' Read from FILE. The default is to read from standard input. `-o FILE' Write to FILE. The default is to write to standard output. `--reverse' Swap the input and output encodings. `--help' Print a help message, then exit. `--version' Print version information, then exit.  File: gcj.info, Node: Invoking rmic, Next: Invoking rmiregistry, Prev: Invoking jv-convert, Up: Top Invoking rmic ************* `rmic' [`OPTION'] ... CLASS ... `rmic' is a utility included with `libgcj' which generates stubs for remote objects. Note that this program isn't yet fully compatible with the JDK `rmic'. 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' By default, `rmic' deletes intermediate files. Either of these options causes it not to delete such files. `-v1.1' Cause `rmic' to create stubs and skeletons for the 1.1 protocol version. `-vcompat' Cause `rmic' to create stubs and skeletons compatible with both the 1.1 and 1.2 protocol versions. This is the default. `-v1.2' Cause `rmic' to create stubs and skeletons for the 1.2 protocol version. `-nocompile' Don't compile the generated files. `-verbose' Print information about what `rmic' is doing. `-d DIRECTORY' Put output files in DIRECTORY. By default the files are put in the current working directory. `-help' Print a help message, then exit. `-version' Print version information, then exit.  File: gcj.info, Node: Invoking rmiregistry, Next: About CNI, Prev: Invoking rmic, Up: Top Invoking rmiregistry ******************** `rmiregistry' [`OPTION'] ... [PORT] `rmiregistry' starts a remote object registry on the current host. If no port number is specified, then port 1099 is used. `--help' Print a help message, then exit. `--version' Print version information, then exit.  File: gcj.info, Node: About CNI, Next: Resources, Prev: Invoking rmiregistry, Up: Top About CNI ********* This documents CNI, the Cygnus 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). * Menu: * Basic concepts:: Introduction to using CNI. * Packages:: How packages are mapped to C++. * Primitive types:: Handling Java types in C++. * Interfaces:: How Java interfaces map to C++. * Objects and Classes:: C++ and Java classes. * Class Initialization:: How objects are initialized. * Object allocation:: How to create Java objects in C++. * Arrays:: Dealing with Java arrays in C++. * Methods:: Java methods in C++. * Strings:: Information about Java Strings. * Mixing with C++:: How CNI can interoperate with C++. * Exception Handling:: How exceptions are handled. * Synchronization:: Synchronizing between Java and C++. * Invocation:: Starting the Java runtime from C++. * Reflection:: Using reflection from C++.  File: gcj.info, Node: Basic concepts, Next: Packages, Up: About CNI Basic concepts ============== 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 and then must include one header file for each Java class it uses, e.g.: #include #include #include 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. (This manual lists `_Jv_AllocBytes' as an example; CNI should instead provide a `JvAllocBytes' function.) Limitations ----------- 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.  File: gcj.info, Node: Packages, Next: Primitive types, Prev: Basic concepts, Up: About CNI Packages ======== 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 nessary namespace declarations. Leaving out package names ------------------------- 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 refered to only by its class-name within the program text. The same effect can be achieved in C++ like this: using namespace PACKAGE-NAME;  File: gcj.info, Node: Primitive types, Next: Interfaces, Prev: Packages, Up: About CNI Primitive types =============== 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 refering to a Java type You should always use these C++ typenames (e.g.: `jint') to avoid disappointment. Reference types associated with primitive types ----------------------------------------------- In Java each primitive type has an associated reference type, e.g.: `boolean' has an associated `java.lang.Boolean' class. In order to make working with such classes easier GCJ provides the macro `JvPrimClass': - macro: JvPrimClass type Return a pointer to the `Class' object corresponding to the type supplied. JvPrimClass(void) => java.lang.Void.TYPE  File: gcj.info, Node: Interfaces, Next: Objects and Classes, Prev: Primitive types, Up: About CNI Interfaces ========== 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.  File: gcj.info, Node: Objects and Classes, Next: Class Initialization, Prev: Interfaces, Up: About CNI Objects and Classes =================== Classes ------- 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. Object fields ------------- 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-bite 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 Integer (int i) { this.i = i; } public static zero = new Integer(0); } you can write: #include ; #include ; Int* mult (Int *p, jint k) { if (k == 0) return Int::zero; // Static member access. return new Int(p->i * k); } Access specifiers ----------------- 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.  File: gcj.info, Node: Class Initialization, Next: Object allocation, Prev: Objects and Classes, Up: About CNI Class Initialization ==================== 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 the G++ compiler 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.) 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 `Jv_InitClass' 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++.  File: gcj.info, Node: Object allocation, Next: Arrays, Prev: Class Initialization, Up: About CNI Object allocation ================= 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); - Function: void* _Jv_AllocBytes (jsize SIZE) Allocates SIZE bytes from the heap. The memory is not scanned by the garbage collector but it freed if no references to it are discovered.  File: gcj.info, Node: Arrays, Next: Methods, Prev: Object allocation, Up: About CNI Arrays ====== 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 JArray : public __JArray { T data[0]; public: T& operator[](jint i) { return data[i]; } }; There are a number of `typedef's which correspond to `typedef's from the JNI. Each is the type of an array holding objects of the relevant type: typedef __JArray *jarray; typedef JArray *jobjectArray; typedef JArray *jbooleanArray; typedef JArray *jbyteArray; typedef JArray *jcharArray; typedef JArray *jshortArray; typedef JArray *jintArray; typedef JArray *jlongArray; typedef JArray *jfloatArray; typedef JArray *jdoubleArray; - Method on template: T* elements (JArray ARRAY) 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 an `int[]' like so: extern jintArray foo; jint *intp = elements (foo); The name of this function may change in the future. - Function: jobjectArray JvNewObjectArray (jsize LENGTH, jclass KLASS, jobject INIT) Here `klass' is the type of elements of the array and `init' is the initial value put into every slot in the array. Creating arrays --------------- 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: - Function: jbooleanArray JvNewBooleanArray (jint LENGTH) Create's an array LENGTH indices long. - Function: jsize JvGetArrayLength (jarray ARRAY) Returns the length of the ARRAY.  File: gcj.info, Node: Methods, Next: Strings, Prev: Arrays, Up: About CNI Methods ======= 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. Overloading ----------- 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 methods -------------- 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::getInteger(jstring str) { ... } Object Constructors ------------------- 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. Instance methods ---------------- 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 jdouble java::lang:Integer::doubleValue() { return (jdouble) value; } Interface methods ----------------- In Java you can call a method using an interface reference. This is supported, but not completly. *Note Interfaces::.  File: gcj.info, Node: Strings, Next: Mixing with C++, Prev: Methods, Up: About CNI Strings ======= CNI provides a number of utility functions for working with Java Java `String' objects. The names and interfaces are analogous to those of JNI. - Function: jstring JvNewString (const char* CHARS, jsize LEN) Returns a Java `String' object with characters from the C string CHARS up to the index LEN in that array. - Function: jstring JvNewStringLatin1 (const char* BYTES, jsize LEN) Returns a Java `String' made up of LEN bytes from BYTES. - Function: jstring JvNewStringLatin1 (const char* BYTES) As above but the length of the `String' is `strlen(BYTES)'. - Function: jstring JvNewStringUTF (const char* BYTES) Returns a `String' which is made up of the UTF encoded characters present in the C string BYTES. - Function: jchar* JvGetStringChars (jstring STR) Returns a pointer to an array of characters making up the `String' STR. - Function: int JvGetStringUTFLength (jstring STR) Returns the number of bytes required to encode the contents of the `String' STR in UTF-8. - Function: jsize JvGetStringUTFRegion (jstring STR, jsize START, jsize LEN, char* BUF) Puts the UTF-8 encoding of a region of the `String' STR into the buffer `buf'. 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.  File: gcj.info, Node: Mixing with C++, Next: Exception Handling, Prev: Strings, Up: About CNI Interoperating with C/C++ ========================= 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 = ... . . . } But this restriction can cause a problem 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 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); }  File: gcj.info, Node: Exception Handling, Next: Synchronization, Prev: Mixing with C++, Up: About CNI Exception Handling ================== 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.  File: gcj.info, Node: Synchronization, Next: Invocation, Prev: Exception Handling, Up: About CNI Synchronization =============== 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 otherwords, you need to manually add `JvSynchronize' in a `native synchornized' method.  File: gcj.info, Node: Invocation, Next: Reflection, Prev: Synchronization, Up: About CNI Invocation ========== 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. - Function: jint JvCreateJavaVM (void* VM_ARGS) 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 call `JvCreateJavaVM()' 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 be `NULL'. This function returns `0' upon success, or `-1' if the runtime is already initialized. _Note:_ In GCJ 3.1, the `vm_args' parameter is ignored. It may be used in a future release. - Function: java::lang::Thread* JvAttachCurrentThread (jstring NAME, java::lang::ThreadGroup* GROUP) 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 be `NULL', in which case a name will be generated. GROUP is the ThreadGroup in which this thread will be a member. If it is `NULL', the thread will be a member of the main thread group. The return value is the Java `Thread' object that represents the thread. It is safe to call `JvAttachCurrentThread()' more than once from the same thread. If the thread is already attached, the call is ignored and the current thread object is returned. - Function: jint JvDetachCurrentThread () 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 returns `0' upon success, or `-1' if the current thread is not attached. Handling uncaught exceptions ---------------------------- 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. Example ------- 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++ test.cc -lgcj'. // test.cc #include #include #include #include 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(); } }  File: gcj.info, Node: Reflection, Prev: Invocation, Up: About CNI Reflection ========== 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.  File: gcj.info, Node: Resources, Prev: About CNI, Up: Top Resources ********* 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.