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This section illustrates how to declare a native method in Java and how to generate the corresponding C/C++ function prototype.
Our first example,
Prompt.java
, contains a
native method that accepts and prints a Java string. The program calls the native method, which waits for
user input and then returns the line the user typed in.
The Prompt
class contains a main
method, which is
used to invoke the program, and a native method named getLine
, which is declared as follows:
private native String getLine(String prompt);
Notice that the declarations for native methods are almost identical to the declarations for regular, non-native Java methods. However, you declare native methods differently, as follows:
native
keyword. The native
keyword informs
the Java compiler that the implementation for this method is provided
in another language.
Compile the Prompt.java
file and then generate
the .h
file. First, compile the Prompt.java
file as follows:
javac Prompt.java
Once you have successfully compiled Prompt.java
and
have created the Prompt.class
file, you can generate a
JNI-style header file by specifying a -jni
option to javah
:
javah -jni Prompt
Examine the Prompt.h
file. Note the
function prototype for the native method getLine
that you
declared in
Prompt.java
.
JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *, jobject, jstring);
The native method function definition in the implementation code
must match the generated function signature in the header file. Always
include JNIEXPORT
and JNICALL
in your native
method function signatures. JNIEXPORT
and
JNICALL
ensure that the source code compiles on platforms
such as Microsoft Windows that require special keywords for functions exported
from dynamic link libraries.
Native method names are concatenated from the following components:
Java_
Graphically, this looks as follows:
Thus, the native code implementation for the Prompt.getLine
method becomes Java_Prompt_getLine
. (Remember that no
package name component appears because the Prompt
class is in the
default package.)
Note: Overloaded native method names, in addition to the above components, have an extra two underscores "__" appended to the method name followed by the argument signature. To illustrate, we created a second version of the Java program,Prompt2.java
, and overloaded thegetLine
method by adding a second argument of typeint
. The twogetLine
method names in thePrompt2.h
header file look as follows:Java_Prompt2_getLine__Ljava_lang_String_2 Java_Prompt2_getLine__Ljava_lang_String_2I
Recall from Step 3: Create the .h File, each native method has two parameters in addition to
those parameters that you declared on the Java side. The first parameter,
JNIEnv *
, is the JNI interface pointer. This interface pointer is organized
as a function table, with every JNI function at a known table
entry point. Your native method invokes specific JNI functions to access
Java objects through the JNIEnv *
pointer. The
jobject
parameter is a reference to the object itself (it
is like the this
pointer in Java).
Lastly, notice that the JNI has a set of type names, such as
jobject
and jstring
, and each type corresponds to Java
types. This is covered in
the next section.
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