Android Native(JNI)層的多線程安全_android native方法是线程安全的吗-优快云博客

本文详细介绍了Java Native 层如何创建子线程，并阐述了多线程之间的线程安全问题。通过具体示例，展示了如何在Native层使用pthread_create()函数来创建子线程，并讨论了线程安全问题的解决方法，如使用monitor指令来实现同步。

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一、Native 程式如何誕生子線程

在Java層的各進程(Process) 都有主線程(Main Thread)，各線程皆可能誕生子線程。這些Java層的線程都有可能並行呼叫同一個Native函數，因而Native函數的線程安全考量是必要的。

Java線程一旦進入Native函數裡執行，在其執行過程中，也可能會誕生子線程，也可能多個線程並行執行同一個非Native的一般C/C++函數。該如何面對他們之間的線程安全問題呢？先談談如何誕生Native子線程呢? 請看範例：

/* com_misoo_thread_JTX05.cpp */

#include "com_misoo_thread_JTX05.h"

#include <utils/Log.h>

#include <utils/IPCThreadState.h>

#include <utils/ProcessState.h>

#include "android_runtime/AndroidRuntime.h"

using namespace android;

JavaVM *gJavaVM;

jmethodID mid;

jclass mClass; // Reference to JTX05 class

jobject mObject; // Weak ref to JTX05 Java object to call on

char sTid[20];

unsigned int e1;

int x;

int sum;

pthread_t thread;

void* trRun( void* );

typedef multimap<string, JNIGlobalRef<jobject> *>MapOfObjects;

JNIEXPORT jstring JNICALL

Java_com_misoo_thread_JTX05_execute(JNIEnv *env, jobject thiz,jobject syncObj){

………

int rr = pthread_create( &thread, NULL,trRun, NULL);

………

}

void* trRun( void* )

{

………

}

使用函數pthread_create()函數來誕生Native層的子線程。

二、多個Native線程的安全問題

由於Native函數裡執行的線程也能誕生子線程，所以也應該注意其線程安全問題。例如：

// JTX07.java

package com.misoo.thread;

import java.lang.ref.WeakReference;

import com.misoo.pk01.ac01;

import android.os.Handler;

import android.os.Message;

import android.os.Process;

public class JTX07 {

private long refer;

private static Handler h;

private static int count;

static {

System.loadLibrary("JTX07_jni");

}

public JTX07(){

init(new WeakReference<JTX07>(this));

count = 0;

h = new Handler(){

public void handleMessage(Message msg) {

count++;

if(count == 1) {

ac01.ref.setTitle("env: " +String.valueOf(msg.arg1));

}

else if(count == 2)

ac01.tv.setText("env: " +String.valueOf(msg.arg1));

}

};

}

public long calculate(){

execute(this);

//this.ssetTitle(ss);

//ac01.ref.setTitle("ss:" + ss);

return 0;

}

@SuppressWarnings("unused")

private static voidcallback(int a, int b){

Message m = h.obtainMessage(1, a, 3,null);

h.sendMessage(m);

}

private native voidinit(Object weak_this);

private native Stringexecute(Object oSync);

//private native Stringexecute(int x);

}

/* com_misoo_thread_JTX07.h */

/* DO NOT EDIT THIS FILE - it is machine generated */

#include <jni.h>

/* Header for class com_misoo_thread_JTX07 */

#ifndef _Included_com_misoo_thread_JTX07

#define _Included_com_misoo_thread_JTX07

#ifdef __cplusplus

extern "C" {

#endif

* Class: com_misoo_thread_JTX07

* Method: nativeSetup

* Signature:(Ljava/lang/Object;)V

void JNICALL Java_com_misoo_thread_Init

(JNIEnv *,jobject, jobject);

* Class: com_misoo_thread_JTX07

* Method: execute

* Signature:(J)J

jstring JNICALL Java_com_misoo_thread_Exec

(JNIEnv *,jobject, jobject);

#ifdef __cplusplus

}

#endif

/* com_misoo_thread_JTX07.cpp */

#include "com_misoo_thread_JTX07.h"

#include <utils/Log.h>

#include <utils/IPCThreadState.h>

#include <utils/ProcessState.h>

#include "android_runtime/AndroidRuntime.h"

using namespace android;

JavaVM *gJavaVM;

jmethodID mid;

jclass mClass; // Reference to JTX07class

jobject mObject; // Weak ref to JTX07 Javaobject to call on

char sTid[20];

unsigned int e1;

int x;

int sum;

pthread_t thread;

void* trRun( void* );

void callBack(JNIEnv *);

jobject mSyncObj;

//typedef multimap<string,JNIGlobalRef<jobject> *> MapOfObjects;

//--------------------------------------------------------------

void Thread_sleep(int t)

{

timespec ts;

ts.tv_sec = t;

ts.tv_nsec =0;

nanosleep(&ts, NULL);

return;

}

void JNICALL

Java_com_misoo_thread_setUp(JNIEnv *env, jobject thiz,jobject weak_this)

{

jclass clazz =env->GetObjectClass(thiz);

mClass =(jclass)env->NewGlobalRef(clazz);

mObject = env->NewGlobalRef(weak_this);

mid =env->GetStaticMethodID(mClass, "callback","(II)V");

return;

}

jstring JNICALL

Java_com_misoo_thread_Exec(JNIEnv *env, jobject thiz,jobject syncObj){

mSyncObj =env->NewGlobalRef(syncObj);

int rr =pthread_create( &thread, NULL, trRun, NULL);

Thread_sleep(4);

callBack(env);

//-----------------------------------------------------------

long pid =getpid();

sprintf(sTid,"%lu", pid);

jstring ret =env->NewStringUTF(sTid);

return ret;

}

//--------------------------------------------------------------------

void callBack(JNIEnv *env){

env->MonitorEnter(mSyncObj);

sum = 0;

for(int i = 0;i<=10; i++)

{

sum +=i;

Thread_sleep(1);

}

//-----------------------------------------------------------

env->CallStaticVoidMethod(mClass, mid, sum, 666);

//-----------------------------------------------------------

env->MonitorExit(mSyncObj);

}

//--------------------------------------------------------------------

static const char *classPathName ="com/misoo/thread/JTX07";

static JNINativeMethod methods[] = {

{"init", "(Ljava/lang/Object;)V",

(void*)Java_com_misoo_thread_setUp},

{"execute", "(Ljava/lang/Object;)Ljava/lang/String;",

(void *)Java_com_misoo_thread_Exec}

};

* Registerseveral native methods for one class.

static int registerNativeMethods(JNIEnv* env, constchar* 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);

returnJNI_FALSE;

}

returnJNI_TRUE;

}

static int registerNatives(JNIEnv* env)

{

if(!registerNativeMethods(env, classPathName,

methods, sizeof(methods) / sizeof(methods[0]))) {

returnJNI_FALSE;

}

returnJNI_TRUE;

}

//----------------------------------------------------------------------------

jint JNI_OnLoad(JavaVM* vm, void* reserved)

{

JNIEnv *env;

gJavaVM = vm;

int result;

LOGI("JNI_OnLoad called");

if (vm->GetEnv((void**) &env, JNI_VERSION_1_4)!= JNI_OK) {

LOGE("Failed to get the environment using GetEnv()");

return -1;

}

if (registerNatives(env) != JNI_TRUE) {

LOGE("ERROR: registerNatives failed");

gotobail;

}

result =JNI_VERSION_1_4;

bail:

returnresult;

}

//--------------------------------------------------------------------------------

void* trRun( void* )

{

int status;

JNIEnv *env;

boolisAttached = false;

Thread_sleep(1);

status =gJavaVM->GetEnv((void **) &env, JNI_VERSION_1_4);

if(status< 0) {

LOGE("callback_handler: failed to get JNI environment, "

"assuming native thread");

status =gJavaVM->AttachCurrentThread(&env, NULL);

if(status< 0) {

LOGE("callback_handler: failed to attach "

"current thread");

return NULL;

}

isAttached = true;

}

//-----------------------------------------------------------

callBack(env);

//-----------------------------------------------------------

if(isAttached)

gJavaVM->DetachCurrentThread();

return NULL;

}

主線程誕生了子線程去執行trRun()函數。必須先調用：

gJavaVM->AttachCurrentThread(&env, NULL);

才能取得子線程自己所屬的JNIEnv物件之參考了，並且呼叫Callback()函數。之後，主線程也呼叫同一Callback函數。於是，在Callback()函數裡，使用env->MonitorEnter()和env->MonitorExit(mSyncObj);指令來讓各線程能達到同步。