Accessing JNI services

The JNI.cpy copybook contains these definitions:

• COBOL data definitions that correspond to the Java JNI types
• JNINativeInterface, the JNI environment structure that contains function pointers for accessing the callable service functions

You obtain the JNI environment structure by two levels of indirection from the JNI environment pointer, as the following illustration shows:

Use the special register JNIEnvPtr to reference the JNI environment pointer to obtain the address for the JNI environment structure. JNIEnvPtr is implicitly defined as USAGE POINTER; do not use it as a receiving data item. Before you reference the contents of the JNI environment structure, you must code the following statements to establish its addressability:

Linkage section.
COPY JNI
. . .
Procedure division.
    Set address of JNIEnv to JNIEnvPtr
    Set address of JNINativeInterface to JNIEnv
    . . .

The code above sets the addresses of the following items:

• JNIEnv, a pointer data item that JNI.cpy provides. JNIEnvPtr is the COBOL special register that contains the environment pointer.
• JNINativeInterface, the COBOL group structure that JNI.cpy contains. This structure maps the JNI environment structure, which contains an array of function pointers for the JNI callable services.

After you code the statements above, you can access the JNI callable services with CALL statements that reference the function pointers. You can pass the JNIEnvPtr special register as the first argument to the services that require the environment pointer, as shown in the following example:

01 InputArrayObj usage object reference jlongArray.
01 ArrayLen pic S9(9) comp-5.
. . .
    Call GetArrayLength using by value JNIEnvPtr InputArrayObj
      returning ArrayLen

Some JNI callable services require a Java class-object reference as an argument. To obtain a reference to the class object that is associated with a class, use one of the following JNI callable services:

• GetObjectClass
• FindClass