jni如何判断两个jobject是否为同一个java对象

jni开发时有时候需要将java对象缓存到native层，方便native层通过jni的反射方法进行回调操作。通常我们会将回调接口callback在native层存放为global reference全局引用，熟悉jni开发的都知道，jni传入到native 层的jobject生命周期仅仅是函数的生命周期，当jni函数返回后对应的jobject对象就会失效，不能再操作，所以就必须申请为global reference。

NewGlobalRef 将变量申请为全局引用，此时java虚拟机会保留jobject所指向的对象防止被垃圾回收器回收。 DeleteGlobalRef 释放全局引用，允许java虚拟机回收该引用指向的java对象，对于不需要使用的global reference必须调用该方法，否则会引起java虚拟机内存泄漏。

既然global reference的实质是指向java对象，那么我们在将某个变量声明为global reference时如何判断该jobject是否已经是当前设置的global reference呢。经过一番查询，看到了一下方法

IsSameObject 如果两个jobject指向同一个java 对象那么返回true，否则返回false。

既然有方法了，接下来就是验证工作了。

Hello.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Hello */#ifndef _Included_Hello
#define _Included_Hello
#ifdef __cplusplus
extern "C" {
#endif
/** Class:     Hello* Method:    native_setObjectToGlobalRef* Signature: (LHello/Book;)V*/
JNIEXPORT void JNICALL Java_Hello_native_1setObjectToGlobalRef(JNIEnv *, jclass, jobject);/** Class:     Hello* Method:    native_release* Signature: ()V*/
JNIEXPORT void JNICALL Java_Hello_native_1release(JNIEnv *, jclass);#ifdef __cplusplus
}
#endif
#endif

Hello.cpp

#ifdef __cplusplus
extern "C" {
#endif#include <cstdio>
#include "Hello.h"// 全局变量，用于存储全局引用的
jobject g_book_ref = NULL; /** Class:     Hello* Method:    native_setObjectToGlobalRef* Signature: (LHello/Book;)V*/
JNIEXPORT void JNICALL Java_Hello_native_1setObjectToGlobalRef(JNIEnv *env, jclass jclazz, jobject jbook) {// 如果输入参数为null，那么不需要继续操作if (NULL == jbook) {printf("native_setObjectToGlobalRef jbook is NUll and g_book_ref is NOT NULL. we need to delete reference of old g_book_ref\n");if (NULL != g_book_ref) {env->DeleteGlobalRef(g_book_ref);  // 删除老的全局引用g_book_ref = NULL;}return;}// 如果全局引用已经设置了，那么需要判断jni函数参数的jbook是否与当前的全局引用指向同一个java对象if (NULL != g_book_ref) {// 如果指向同一个对象，那么不需要再次为该jbook申请全局引用if (env->IsSameObject(g_book_ref, jbook)) {printf("native_setObjectToGlobalRef isSameObject: true, g_book_ref: %p, jbook: %p\n", g_book_ref, jbook);} else {// 如果指向不同对象，那么先释放老的全局引用，再为jbook申请全局引用printf("native_setObjectToGlobalRef isSameObject: false\n");printf("native_setObjectToGlobalRef g_book_ref and jbook is not same object. we need to delete reference of old g_book_ref\n");env->DeleteGlobalRef(g_book_ref);printf("native_setObjectToGlobalRef create global reference to g_book_ref\n");g_book_ref = env->NewGlobalRef(jbook);}} else {printf("native_setObjectToGlobalRef g_book_ref is NULL\n");printf("native_setObjectToGlobalRef create global reference to g_book_ref\n");g_book_ref = env->NewGlobalRef(jbook);}
}/** Class:     Hello* Method:    native_release* Signature: ()V*/
JNIEXPORT void JNICALL Java_Hello_native_1release(JNIEnv *env, jclass jclazz) {printf("native_release g_book_ref: %p\n", g_book_ref);if (NULL != g_book_ref) {env->DeleteGlobalRef(g_book_ref);g_book_ref = NULL;}
}#ifdef __cplusplus
}
#endif

Hello.java

public class Hello {static {try {String sysName = System.getProperty("os.name");if (sysName.contains("Linux")) {System.load(System.getProperty("user.dir") + "/libHello.so");} else if (sysName.contains("Drawin") || sysName.contains("Mac")) {System.load(System.getProperty("user.dir") + "/libHello.dylib");}} catch (java.lang.UnsatisfiedLinkError e) {e.printStackTrace();System.err.println("load so failed.");System.exit(1);}}static class Book {public String name;public Book() {}public Book(String name) {this.name = name;}}public native static void native_setObjectToGlobalRef(Book book);public native static void native_release();public static void main(String args[]) {Book b1 = new Book();Book b2 = new Book();System.out.println("set b1");native_setObjectToGlobalRef(b1);System.out.println("set b2");native_setObjectToGlobalRef(b2);System.out.println("##set b2");native_setObjectToGlobalRef(b2);System.out.println("##set null");native_setObjectToGlobalRef(null);native_release();}
}

makefile

UNAME_S := $(shell uname -s)
ifeq ($(UNAME_S),Darwin)
main:gcc -I"${JAVA_HOME}/include" -I"${JAVA_HOME}/include/darwin" Hello.cpp -dynamiclib -o libHello.dylib
endififeq ($(UNAME_S),Linux)
main:gcc -I"${JAVA_HOME}/include" -I"${JAVA_HOME}/include/linux" Hello.cpp -shared -fPIC -o libHello.so
endif

运行命令

$ make

$ javac Hello.java

运行程序

$ java -Djava.library.path=".:${JAVA_HOME}" Hello

输出结果

set b1
native_setObjectToGlobalRef g_book_ref is NULL
native_setObjectToGlobalRef create global reference to g_book_ref
set b2
native_setObjectToGlobalRef isSameObject: false
native_setObjectToGlobalRef g_book_ref and jbook is not same object. we need to delete reference of old g_book_ref
native_setObjectToGlobalRef create global reference to g_book_ref
##set b2
native_setObjectToGlobalRef isSameObject: true, g_book_ref: 0x7fb16ae08e90, jbook: 0x70000ce4a900
##set null
native_setObjectToGlobalRef jbook is NUll and g_book_ref is NOT NULL. we need to delete reference of old g_book_ref
native_release g_book_ref: 0x0

1. set b1时

因为是首次设置，此时native的g_book_ref还是null，没有被指向java对象，所以需要用全局引用为jbook保存java对象引用；

2. set b2时

isSameObject返回false，证明g_book_ref和jbook指向的对象不一样（此时jbook指向b2，而g_book_ref指向步骤1设置的b1），所以需要先删除老的b1全局引用再为b2申请全局引用；

3. 再次设置b2时

isSameObject返回true，证明g_book_ref和jbook指向的对象一样（此时jbook和g_book_ref都指向b2，注意两者的native地址不一样），为了节省操作，所以不需要在为jbook申请全局引用；

4. 设置为null

因为jbook为null，所以需要释放全局引用g_book_ref。

5. native释放

在Book native释放时因为步骤4已经提前释放了全局引用，所以不需要再释放了。