Android uses JNI to call C ++ custom classes

I have been searching for articles about android Calling C/C ++ libraries for a long time, but there is no article about android Calling C ++ custom classes using jni, instead, I had to look at the android source code and learn how to implement the android graphics library, because the underlying implementation of this library also uses the C ++ skia library. The following three files are described.

First, you have a class in java, which is called by the Person class in your application.

[Java]
Package whf. jnitest;
 
Public class Person {

Static
{
System. loadLibrary ("Person ");
}

Private int mNativePerson;

Public Person ()
{
MNativePerson = init ();
}
Protected void finalize ()
{
Try {
Finalizer (mNativePerson );
} Finally {
Try {
Super. finalize ();
} Catch (Throwable e ){
// TODO Auto-generated catch block
E. printStackTrace ();
}
}
}
Public int getAge ()
{
Return native_getAge (mNativePerson );
}

Public void setAge (int age)
{
Native_setAge (mNativePerson, age );
}
 
Private native void finalizer (int nPerson );
Public native int add (int a, int B );
Public native int sub (int a, int B );
Private native int init ();

Private native int native_getAge (int nPerson );
Private native void native_setAge (int nPerson, int age );
}

Then your C ++ file is as follows. I define it as the CPerson class.
[Cpp]
# Define LOG_TAG "this is a first JNI test"
 
# Include <stdio. h>
# Include <stdlib. h>
# Include <unistd. h>
# Include <sys/types. h>
# Include <sys/stat. h>
# Include <fcntl. h>
# Include <assert. h>
# Include "jni. h"
# Include "CPerson. h"
 
 
 
Jint add (JNIEnv * env, jobject obj, jint a, jint B)
{
Return a + B;
}
 
Jint Java_whf_jnitest_Person_sub (JNIEnv * env, jobject obj, jint a, jint B)
{
Return a-B;
}
 
Void Java_whf_jnitest_Person_finalizer (JNIEnv * env, jobject clazz, CPerson * obj)
{
Delete obj;
}
 
CPerson * Java_whf_jnitest_Person_init (JNIEnv * env, jobject clazz)
{
CPerson * p = new CPerson ();
Return p;
}
 
Void Java_whf_jnitest_Person_setAge (JNIEnv * env, jobject clazz, CPerson * obj, jint age)
{
Return obj-> setAge (age );
}
 
Jint Java_whf_jnitest_Person_getAge (JNIEnv * env, jobject clazz, CPerson * obj)
{
Return obj-> getAge ();
}
 
Static JNINativeMethod methods [] {
{"Add", "(II) I", (void *) add },
{"Sub", "(II) I", (void *) Java_whf_jnitest_Person_sub },
{"Finalizer", "(I) V", (void *) Java_whf_jnitest_Person_finalizer },
{"Init", "() I", (void *) Java_whf_jnitest_Person_init },
{"Native_setAge", "(II) V", (void *) Java_whf_jnitest_Person_setAge },
{"Native_getAge", "(I) I", (void *) Java_whf_jnitest_Person_getAge },
};
 
Static const char * classPathName = "whf/jnitest/Person ";
 
Static int registerNativeMethods (JNIEnv * env, const char * className,
JNINativeMethod * gMethods, int numMethods)
{
Jclass clazz;
 
Clazz = env-> FindClass (className );
If (clazz = NULL ){
// LOGE ("Native registration unable to find class '% S'", className );
Return JNI_FALSE;
}
If (env-> RegisterNatives (clazz, gMethods, numMethods) <0 ){
// LOGE ("RegisterNatives failed for '% S'", className );
Return JNI_FALSE;
}
 
Return JNI_TRUE;
}
 
Static int registerNatives (JNIEnv * env)
{
If (! RegisterNativeMethods (env, classPathName,
Methods, sizeof (methods)/sizeof (methods [0]) {
Return JNI_FALSE;
}
 
Return JNI_TRUE;
}
 
Typedef union {
JNIEnv * env;
Void * venv;
} UnionJNIEnvToVoid;
 
/* This function will be call when the library first be loaded */
Jint JNI_OnLoad (JavaVM * vm, void * reserved)
{
UnionJNIEnvToVoid uenv;
JNIEnv * env = NULL;
// LOGI ("JNI_OnLoad! ");
 
If (vm-> GetEnv (void **) & uenv. venv, JNI_VERSION_1_4 )! = JNI_ OK ){
// LOGE ("ERROR: GetEnv failed ");
Return-1;
}
 
Env = uenv. env ;;
 
// JniRegisterNativeMethods (env, "whf/jnitest/Person", methods, sizeof (methods)/sizeof (methods [0]);
 
If (registerNatives (env )! = JNI_TRUE ){
// LOGE ("ERROR: registerNatives failed ");
Return-1;
}
 
Return JNI_VERSION_1_4;
}

CPerson's statement is as follows:
[Cpp]
# Ifndef _ CPERSON_H _
# Define _ CPERSON_H _
 
Class CPerson
{
Private:
Int m_age;
Public:
CPerson ();
Int getAge ();
Void setAge (int age );
};
 
# Endif www.2cto.com

So a simple JNI calls the C ++ custom class.

Author: wanghaofeng