The Java Native Interface: Calling a native method

The Java Native Interface

JNI is a fairly rich programming interface that allows you to call native methods from a Java application. It was added in Java 1.1, maintaining a certain degree of compatibility with its Java 1.0 equivalent, the native method interface (NMI). NMI has design characteristics that make it unsuitable for adoption in all virtual machines. For this reason, future versions of the language might no longer support NMI, and it will not be covered here.

Currently, JNI is designed to interface with native methods written only in C or C++. Using JNI, your native methods can:

Create, inspect, and update Java objects (including arrays and Strings)

Call Java methods

Catch and throw exceptions

Load classes and obtain class information

Perform runtime type checking

Thus, virtually everything you can do with classes and objects in ordinary Java you can also do in native methods.

Calling a native method

We'll start with a simple example: a Java program that calls a native method, which in turn calls the Win32 MessageBox( ) API function to display a graphical text box. This example will also be used later with J/Direct. If your platform is not Win32, just replace the C header include:

with

and replace the call to MessageBox( ) with a call to printf( ).

The first step is to write the Java code declaring a native method and its arguments:

The native method declaration is followed by a static block that calls System.loadLibrary( ) (which you could call at any time, but this style is more appropriate). System.loadLibrary( ) loads a DLL in memory and links to it. The DLL must be in your system path or in the directory containing the Java class file. The file name extension is automatically added by the JVM depending on the platform.

The C header file generator: javah

Now compile your Java source file and run javah on the resulting .class file. Javah was present in version 1.0, but since you are using Java 1.1 JNI you must specify the -jni switch:

Javah reads the Java class file and for each native method declaration it generates a function prototype in a C or C++ header file. Here's the output: the ShowMsgBox.h source file (edited slightly to fit into the book):

As you can see by the #ifdef __cplusplus preprocessor directive, this file can be compiled either by a C or a C++ compiler. The first #include directive includes jni.h, a header file that, among other things, defines the types that you can see used in the rest of the file. JNIEXPORT and JNICALL are macros that expand to match platform-specific directives; JNIEnv, jobject and jstring are JNI data type definitions.

Name mangling and function signatures

JNI imposes a naming convention (called name mangling) on native methods; this is important, since it's part of the mechanism by which the virtual machine links Java calls to native methods. Basically, all native methods start with the word "Java," followed by the name of the class in which the Java native declaration appears, followed by the name of the Java method; the underscore character is used as a separator. If the Java native method is overloaded, then the function signature is appended to the name as well; you can see the native signature in the comments preceding the prototype. For more information about name mangling and native method signatures, please refer to the JNI documentation.

Implementing your DLL

At this point, all you have to do is write a C or C++ source file that includes the javah-generated header file and implements the native method, then compile it and generate a dynamic link library. This part is platform-dependent, and I'll assume that you know how to create a DLL. The code below implements the native method by calling a Win32 API. It is then compiled and linked into a file called MsgImpl.dll (for "Message Implementation").

If you have no interest in Win32, just skip the MessageBox( ) call; the interesting part is the surrounding code. The arguments that are passed into the native method are the gateway back into Java. The first, of type JNIEnv, contains all the hooks that allow you to call back into the JVM. (We'll look at this in the next section.) The second argument has a different meaning depending on the type of method. For non-static methods like the example above (also called instance methods), the second argument is the equivalent of the "this" pointer in C++ and similar to this in Java: it's a reference to the object that called the native method. For static methods, it's a reference to the Class object where the method is implemented.

The remaining arguments represent the Java objects passed into the native method call. Primitives are also passed in this way, but they come in by value.

In the following sections we'll explain this code by looking at how to access and control the JVM from inside a native method.

Accessing JNI functions:
The JNIEnv argument

JNI functions are those that the programmer uses to interact with the JVM from inside a native method. As you can see in the example above, every JNI native method receives a special argument as its first parameter: the JNIEnv argument, which is a pointer to a special JNI data structure of type JNIEnv_. One element of the JNI data structure is a pointer to an array generated by the JVM; each element of this array is a pointer to a JNI function. The JNI functions can be called from the native method by dereferencing these pointers (it's simpler than it sounds). Every JVM provides its own implementation of the JNI functions, but their addresses will always be at predefined offsets.

Through the JNIEnv argument, the programmer has access to a large set of functions. These functions can be grouped into the following categories:

Obtaining version information

Performing class and object operations

Handling global and local references to Java objects

Accessing instance fields and static fields

Calling instance methods and static methods

Performing string and array operations

Generating and handling Java exceptions

The number of JNI functions is quite large and won't be covered here. Instead, I'll show the rationale behind the use of these functions. For more detailed information, consult your compiler's JNI documentation.

If you take a look at the jni.h header file, you'll see that inside the #ifdef __cplusplus preprocessor conditional, the JNIEnv_ structure is defined as a class when compiled by a C++ compiler. This class contains a number of inline functions that let you access the JNI functions with an easy and familiar syntax. For example, the line in the preceding example

can be rewritten as follows in C++:

You'll notice that you no longer need the double dereferencing of the jEnv pointer, and that the same pointer is no longer passed as the first parameter to the JNI function call. In the rest of these examples, I'll use the C++ style.

Accessing Java Strings

As an example of accessing a JNI function, consider the code shown above. Here, the JNIEnv argument jEnv is used to access a Java String. Java Strings are in Unicode format, so if you receive one and want to pass it to a non-Unicode function (printf( ), for example), you must first convert it into ASCII characters with the JNI function GetStringUTFChars( ). This function takes a Java String and converts it to UTF-8 characters. (These are 8 bits wide to hold ASCII values or 16 bits wide to hold Unicode. If the content of the original string was composed only of ASCII, the resulting string will be ASCII as well.)

GetStringUTFChars is the name of one of the fields in the structure that JNIEnv is indirectly pointing to, and this field in turn is a pointer to a function. To access the JNI function, we use the traditional C syntax for calling a function though a pointer. You use the form above to access all of the JNI functions.

Passing and using Java objects

In the previous example we passed a String to the native method. You can also pass Java objects of your own creation to a native method. Inside your native method, you can access the fields and methods of the object that was received.

To pass objects, use the ordinary Java syntax when declaring the native method. In the example below, MyJavaClass has one public field and one public method. The class UseObjects declares a native method that takes an object of class MyJavaClass. To see if the native method manipulates its argument, the public field of the argument is set, the native method is called, and then the value of the public field is printed.

After compiling the code and handing the .class file to javah, you can implement the native method. In the example below, once the field and method ID are obtained, they are accessed through JNI functions.

The first argument aside, the C++ function receives a jobject, which is the native side of the Java object reference we pass from the Java code. We simply read aValue, print it out, change the value, call the object's divByTwo( ) method, and print the value out again.

To access a field or method, you must first obtain its identifier. Appropriate JNI functions take the class object, the element name, and the signature. These functions return an identifier that you use to access the element. This approach might seem convoluted, but your native method has no knowledge of the internal layout of the Java object. Instead, it must access fields and methods through indexes returned by the JVM. This allows different JVMs to implement different internal object layouts with no impact on your native methods.

If you run the Java program, you'll see that the object that's passed from the Java side is manipulated by your native method. But what exactly is passed? A pointer or a Java reference? And what is the garbage collector doing during native method calls?

The garbage collector continues to operate during native method execution, but it's guaranteed that your objects will not be garbage collected during a native method call. To ensure this, local references are created before, and destroyed right after, the native method call. Since their lifetime wraps the call, you know that the objects will be valid throughout the native method call.

Since these references are created and subsequently destroyed every time the function is called, you cannot make local copies in your native methods, in static variables. If you want a reference that lasts across function invocations, you need a global reference. Global references are not created by the JVM, but the programmer can make a global reference out of a local one by calling specific JNI functions. When you create a global reference, you become responsible for the lifetime of the referenced object. The global reference (and the object it refers to) will be in memory until the programmer explicitly frees the reference with the appropriate JNI function. It's similar to malloc( ) and free( ) in C.

JNI and Java exceptions

With JNI, Java exceptions can be thrown, caught, printed, and rethrown just as they are inside a Java program. But it's up to the programmer to call dedicated JNI functions to deal with exceptions. Here are the JNI functions for exception handling:

Throw( )
Throws an existing exception object. Used in native methods to rethrow an exception.

ThrowNew( )
Generates a new exception object and throws it.

ExceptionOccurred( )
Determines if an exception was thrown and not yet cleared.

ExceptionDescribe( )
Prints an exception and the stack trace.

ExceptionClear( )
Clears a pending exception.

FatalError( )
Raises a fatal error. Does not return.

Among these, you can't ignore ExceptionOccurred( ) and ExceptionClear( ). Most JNI functions can generate exceptions, and there is no language feature that you can use in place of a Java try block, so you must call ExceptionOccurred( ) after each JNI function call to see if an exception was thrown. If you detect an exception, you may choose to handle it (and possibly rethrow it). You must make certain, however, that the exception is eventually cleared. This can be done in your function using ExceptionClear( ) or in some other function if the exception is rethrown, but it must be done.

You must ensure that the exception is cleared, because otherwise the results will be unpredictable if you call a JNI function while an exception is pending. There are few JNI functions that are safe to call during an exception; among these, of course, are all the exception handling functions.

JNI and threading

Since Java is a multithreaded language, several threads can call a native method concurrently. (The native method might be suspended in the middle of its operation when a second thread calls it.) It's entirely up to the programmer to guarantee that the native call is thread-safe, i.e. it does not modify shared data in an unmonitored way. Basically, you have two options: declare the native method as synchronized or implement some other strategy within the native method to ensure correct, concurrent data manipulation.

Also, you should never pass the JNIEnv pointer across threads, since the internal structure it points to is allocated on a per-thread basis and contains information that makes sense only in that particular thread.

Using a pre-existing code base

The easiest way to implement JNI native methods is to start writing native method prototypes in a Java class, compile that class, and run the .class file through javah. But what if you have a large, pre-existing code base that you want to call from Java? Renaming all the functions in your DLLs to match the JNI name mangling convention is not a viable solution. The best approach is to write a wrapper DLL "outside" your original code base. The Java code calls functions in this new DLL, which in turn calls your original DLL functions. This solution is not just a work-around; in most cases you must do this anyway because you must call JNI functions on the object references before you can use them.