如何在android的jni线程中实现回调

Posted 2023-03-25

JNI回调是指在c/c++代码中调用java函数，当在c/c++的线程中执行回调函数时，会导致回调失败。

其中一种在android系统的解决方案是：

把c/c++中所有线程的创建，由pthread_create函数替换为由Java层的创建线程的函数AndroidRuntime::createJavaThread。

假设有c++函数:

[cpp] view plaincopy
void *thread_entry(void *args)

while(1)

printf("thread running...\n");
sleep(1);




void init()

pthread_t thread;
pthread_create(&thread,NULL,thread_entry,(void *)NULL);


init()函数创建一个线程，需要在该线程中调用java类Test的回调函数Receive:

[cpp] view plaincopy
public void Receive(char buffer[],int length)
String msg = new String(buffer);
msg = "received from jni callback:" + msg;
Log.d("Test", msg);


首先在c++中定义回调函数指针：

[cpp] view plaincopy
//test.h
#include <pthread.h>
//function type for receiving data from native
typedef void (*ReceiveCallback)(unsigned char *buf, int len);

/** Callback for creating a thread that can call into the Java framework code.
* This must be used to create any threads that report events up to the framework.
*/
typedef pthread_t (* CreateThreadCallback)(const char* name, void (*start)(void *), void* arg);

typedef struct
ReceiveCallback recv_cb;
CreateThreadCallback create_thread_cb;
Callback;

再修改c++中的init和thread_entry函数：

[cpp] view plaincopy
//test.c
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/wait.h>
#include <unistd.h>
#include "test.h"

void *thread_entry(void *args)

char *str = "i'm happy now";
Callback cb = NULL;
int len;
if(args != NULL)
cb = (Callback *)args;


len = strlen(str);
while(1)

printf("thread running...\n");
//invoke callback method to java
if(cb != NULL && cb->recv_cb != NULL)
cb->recv_cb((unsigned char*)str, len);

sleep(1);




void init(Callback *cb)

pthread_t thread;
//pthread_create(&thread,NULL,thread_entry,(void *)NULL);
if(cb != NULL && cb->create_thread_cb != NULL)

cb->create_thread_cb("thread",thread_entry,(void *)cb);



然后在jni中实现回调函数，以及其他实现：

[cpp] view plaincopy
//jni_test.c
#include <stdlib.h>
#include <malloc.h>
#include <jni.h>
#include <JNIHelp.h>
#include "android_runtime/AndroidRuntime.h"

#include "test.h"
#define RADIO_PROVIDER_CLASS_NAME "com/tonny/Test"

using namespace android;

static jobject mCallbacksObj = NULL;
static jmethodID method_receive;

static void checkAndClearExceptionFromCallback(JNIEnv* env, const char* methodName)
if (env->ExceptionCheck())
LOGE("An exception was thrown by callback '%s'.", methodName);
LOGE_EX(env);
env->ExceptionClear();



static void receive_callback(unsigned char *buf, int len)

int i;
JNIEnv* env = AndroidRuntime::getJNIEnv();
jcharArray array = env->NewCharArray(len);
jchar *pArray ;

if(array == NULL)
LOGE("receive_callback: NewCharArray error.");
return;


pArray = (jchar*)calloc(len, sizeof(jchar));
if(pArray == NULL)
LOGE("receive_callback: calloc error.");
return;


//copy buffer to jchar array
for(i = 0; i < len; i++)

*(pArray + i) = *(buf + i);

//copy buffer to jcharArray
env->SetCharArrayRegion(array,0,len,pArray);
//invoke java callback method
env->CallVoidMethod(mCallbacksObj, method_receive,array,len);
//release resource
env->DeleteLocalRef(array);
free(pArray);
pArray = NULL;

checkAndClearExceptionFromCallback(env, __FUNCTION__);


static pthread_t create_thread_callback(const char* name, void (*start)(void *), void* arg)

return (pthread_t)AndroidRuntime::createJavaThread(name, start, arg);


static Callback mCallbacks =
receive_callback,
create_thread_callback
;

static void jni_class_init_native
(JNIEnv* env, jclass clazz)

method_receive = env->GetMethodID(clazz, "Receive", "([CI)V");


static int jni_init
(JNIEnv *env, jobject obj)


if (!mCallbacksObj)
mCallbacksObj = env->NewGlobalRef(obj);

return init(&mCallbacks);


static const JNINativeMethod gMethods[] =
"class_init_native", "()V", (void *)jni_class_init_native ,
"native_init", "()I", (void *)jni_init ,
;

static int registerMethods(JNIEnv* env)

const char* const kClassName = RADIO_PROVIDER_CLASS_NAME;
jclass clazz;
/* look up the class */
clazz = env->FindClass(kClassName);
if (clazz == NULL)
LOGE("Can't find class %s/n", kClassName);
return -1;

/* register all the methods */
if (env->RegisterNatives(clazz,gMethods,sizeof(gMethods)/sizeof(gMethods[0])) != JNI_OK)

LOGE("Failed registering methods for %s/n", kClassName);
return -1;

/* fill out the rest of the ID cache */
return 0;


jint JNI_OnLoad(JavaVM* vm, void* reserved)
JNIEnv* env = NULL;
jint result = -1;
LOGI("Radio JNI_OnLoad");
if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK)
LOGE("ERROR: GetEnv failed/n");
goto fail;


if(env == NULL)
goto fail;

if (registerMethods(env) != 0)
LOGE("ERROR: PlatformLibrary native registration failed/n");
goto fail;

/* success -- return valid version number */
result = JNI_VERSION_1_4;
fail:
return result;


jni的Android.mk文件中共享库设置为：

[cpp] view plaincopy
LOCAL_SHARED_LIBRARIES := liblog libcutils libandroid_runtime libnativehelper

最后再实现Java中的Test类：

[java] view plaincopy
//com.tonny.Test.java

public class Test

static
try
System.loadLibrary("test");
class_init_native();

catch(UnsatisfiedLinkError ule)
System.err.println("WARNING: Could not load library libtest.so!");




public int initialize()
return native_radio_init();


public void Receive(char buffer[],int length)
String msg = new String(buffer);
msg = "received from jni callback" + msg;
Log.d("Test", msg);


protected static native void class_init_native();

protected native int native_init();

参考技术A 主线程吧我记得像是XUitls的连网回调函数。在onsuccess和onfailure方法里面，可以直接关闭弹出的联网进度条既然可以修改控件，应该就是在主线程了吧