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:
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:
class ShowMsgBox {
public static void main(String [] args) {
ShowMsgBox app = new ShowMsgBox();
app.ShowMessage("Generated with JNI");
}
private native void ShowMessage(String msg);
static {
System.loadLibrary("MsgImpl");
}
}
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):
/* DO NOT EDIT THIS FILE
- it is machine generated */
#include <jni.h>
/* Header for class ShowMsgBox */
#ifndef _Included_ShowMsgBox
#define _Included_ShowMsgBox
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: ShowMsgBox
* Method: ShowMessage
* Signature: (Ljava/lang/String;)V
*/
JNIEXPORT void JNICALL
Java_ShowMsgBox_ShowMessage
(JNIEnv *, jobject, jstring);
#ifdef __cplusplus
}
#endif
#endif
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”).
#include <windows.h>
#include "ShowMsgBox.h"
BOOL APIENTRY DllMain(HANDLE hModule,
DWORD dwReason, void** lpReserved) {
return TRUE;
}
JNIEXPORT void JNICALL
Java_ShowMsgBox_ShowMessage(JNIEnv * jEnv,
jobject this, jstring jMsg) {
const char * msg;
msg = (*jEnv)->GetStringUTFChars(jEnv, jMsg,0);
MessageBox(HWND_DESKTOP, msg,
"Thinking in Java: JNI",
MB_OK | MB_ICONEXCLAMATION);
(*jEnv)->ReleaseStringUTFChars(jEnv, jMsg,msg);
}
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
(*jEnv)->ReleaseStringUTFChars(jEnv,
jMsg,msg);
can
be rewritten as follows in C++:
jEnv->ReleaseStringUTFChars(jMsg,msg); 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.
class MyJavaClass {
public void divByTwo() { aValue /= 2; }
public int aValue;
}
public class UseObjects {
public static void main(String [] args) {
UseObjects app = new UseObjects();
MyJavaClass anObj = new MyJavaClass();
anObj.aValue = 2;
app.changeObject(anObj);
System.out.println("Java: " + anObj.aValue);
}
private native void
changeObject(MyJavaClass obj);
static {
System.loadLibrary("UseObjImpl");
}
}
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.
JNIEXPORT void JNICALL
Java_UseObjects_changeObject(
JNIEnv * env, jobject jThis, jobject obj) {
jclass cls;
jfieldID fid;
jmethodID mid;
int value;
cls = env->GetObjectClass(obj);
fid = env->GetFieldID(cls,
"aValue", "I");
mid = env->GetMethodID(cls,
"divByTwo", "()V");
value = env->GetIntField(obj, fid);
printf("Native: %d\n", value);
env->SetIntField(obj, fid, 6);
env->CallVoidMethod(obj, mid);
value = env->GetIntField(obj, fid);
printf("Native: %d\n", value);
}
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.