jni jnienv GetByteField example source

Get<type>Field Routines - GetByteField

NativeType Get<type>Field(JNIEnv *env, jobject obj,
jfieldID fieldID);

This family of accessor routines returns the value of an instance (nonstatic) field of an object. The field to access is specified by a field ID obtained by calling GetFieldID(). (GetByteField)

The following table describes the Get<type>Field routine name and result type. You should replace type in Get<type>Field with the Java type of the field, or use one of the actual routine names from the table, and replace NativeType with the corresponding native type for that routine.

Table 4-1a Get<type>Field Family of Accessor Routines

Get<type>Field Routine Name	Native Type
GetObjectField()	jobject
GetBooleanField()	jboolean
GetByteField()	jbyte
GetCharField()	jchar
GetShortField()	jshort
GetIntField()	jint
GetLongField()	jlong
GetFloatField()	jfloat
GetDoubleField()	jdouble

LINKAGE:

Indices in the JNIEnv interface function table:

Table 4-1b Get<type>Field Family of Accessor Routines

Get<type>Field Routine Name	Index
GetObjectField()	95
GetBooleanField()	96
GetByteField()	97
GetCharField()	98
GetShortField()	99
GetIntField()	100
GetLongField()	101
GetFloatField()	102
GetDoubleField()	103

PARAMETERS:

env: the JNI interface pointer.

obj: a Java object (must not be NULL).

fieldID: a valid field ID.

RETURNS:

Returns the content of the field.

Example - GetByteField

native void nativeModifyByte(Byte b);

JNIEXPORT void JNICALL Java_nativeModifyByte(JNIEnv * env, jobject o, jobject b) {
   jclass c = (*env)->GetObjectClass(env, b);
   jfieldID fid = (*env)->GetFieldID(env, c, "value", "B");
   jbyte oldVal = (*env)->GetByteField(env, b, fid);
   (*env)->SetByteField(env, b, fid, 42);
}

Usage:

    byte v = 13;
    Byte b = new Byte(v);
    nativeModifyByte(b);
    System.out.println(b);

Output:

    42