Java methods are not called when called from native pthread

Java methods are not called when called from native pthread … here is a solution to the problem.

Java methods are not called when called from native pthread

I need a simple Java service that starts at system startup and uses shared libraries and some functionality that uses POSIX threads. I ran into some problems with implementing the JNI interface, which did not allow me to call Java methods from native code. GetMethodID() doesn’t return NULL, so I think it works fine. Nor are there any suspicious bugs that can help. So I added a lot of output to the log and prepared a simple Java test for this. (All the code is listed below, but the project can also be found in this repository at github).

List of project files:

Services:

  • Test the service .java
  • Test the Controller .java
  • TestListener.java
  • TestNative .java <

Native Code:

  • layer-jni.c <

Other:

  • Android.mk
  • Apply .mk
  • AndroidManifest.xml

All files are listed below.

I tried the following logic in the test service:

1. Service start:

Test service .java:

package com.example.testservice;

import java.lang.String;

import android.app.Notification;
import android.app.PendingIntent;
import android.app.Service;
import android.content.Intent;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Looper;
import android.os.Message;
import android.util.Log;
import android.widget.Toast;

public class TestService extends Service {
    private Looper mServiceLooper;
    private ServiceHandler mServiceHandler;
    private TestController testCtrl = null;
    private static final String TAG = "TestService";

private final class ServiceHandler extends Handler {
         public ServiceHandler(Looper looper) {
                       super(looper);
         }
         @Override
         public void handleMessage(Message msg) {
                 Log.e(TAG, msg.toString());
                 Log.e(TAG, "IT WORKS");
         }
    }

@Override
    public IBinder onBind(Intent intent) {
        return null;
    }
    public void onDestroy() {
        Toast.makeText(this, "Test service stopped", Toast.LENGTH_LONG).show();
        Log.d(TAG, "Test has been stopped");
    }

@Override
    public void onCreate() {
        super.onCreate();
        Log.i(TAG, "onCreate");
        HandlerThread thread = new HandlerThread("ServiceStartArguments",
                           android.os.Process.THREAD_PRIORITY_BACKGROUND);
        thread.start();

mServiceLooper = thread.getLooper();
        mServiceHandler = new ServiceHandler(mServiceLooper);
    }

@Override
    public void onStart(Intent intent, int startid)
    {         
        if (testCtrl != null) {
            Log.d(TAG, "Service already running.");
            return;
        }
        Log.d(TAG, "Starting test controller");
        testCtrl = new TestController();
        Log.d(TAG, "Test controller has started");
    } 
}

2. It creates the

TestController class (in which most of the logic must be), which creates the TestNative class.

Interface TestListener .java:

package com.example.testservice;

public interface TestListener {
    public void stringJavaMethod(String regStr);
}

TestController.java:

package com.example.testservice;

import android.util.Log;

public class TestController implements TestListener {
    private static final String TAG = "TestController";
    private TestNative mTestNative = null;

TestController() {
        Log.d(TAG, "Starting test native");
        mTestNative = new TestNative(this);
    }

@Override
    public void stringJavaMethod(String regStr) {
        Log.d(TAG, "Callback called!!!! \n");
        Log.e(TAG, regStr);
    }

TestNative.java:

package com.example.testservice;

import android.os.Handler;
import android.util.Log;

public class TestNative implements TestListener {
    static {
        System.loadLibrary("log");
        System.loadLibrary("layer-jni");
    }

private static final String TAG = "TestNative";
    private Handler mHandler;
    private TestListener mDelegate;

TestNative(TestListener t) {
        mDelegate = t;
        mHandler = new Handler();
        startAthread();
    }

@Override
    public void stringJavaMethod(final String regStr) {
        Log.d(TAG, "IT WORKS?" + regStr);
        mHandler.post(new Runnable() {
            public void run() {
                Log.e(TAG, "CALLED!\n");
                mDelegate.stringJavaMethod(regStr);
            }
        });
    }    

/* native interface */
    public static native void startAthread();
}

3.The TestNative class requests the local library to start working through the startAthread() method.
4.Native code stores the JVM, makes a global reference to the calling object, and starts a thread.
5.Thread attaches itself to the JVM and gets a new JNIEnv* pointer. It then looks up the Java method ID using the global object link obtained in step #4 and then tries to call this method periodically.

The only native source is layer-jni.c:

#include "logcat.h"
#include "layer-jni.h"

#include <jni.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

#include <android/asset_manager.h>
#include <android/asset_manager_jni.h>

JavaVM *jvm = NULL;
/* removed jObj UPD3 */
 jobject jObj;
/* added see UPD2 */
jclass jCls;
/******************/

static int run = 0;
static pthread_t t;
/* jobject -> jclass */
void callVoidMethodString(JNIEnv *env, jclass jcl, jmethodID jmid, const char *str);

JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *ajvm, void *dummy) {
        return JNI_VERSION_1_6;
}

void *thread_func(void *dummy) {
        run = 1;
        JNIEnv *env = NULL;
        if (JNI_EDETACHED == (*jvm)->GetEnv(jvm, (void**)&env, JNI_VERSION_1_6)) {
                if ( 0 != (*jvm)->AttachCurrentThread(jvm, &env, NULL)) {
                        DBG_INFO("Cannot attach JNIEnv!\n");
                }
        }
        /* UPD2 - jObj -> jCls */
        jmethodID jmid = (*env)->GetMethodID(env, jCls, "stringJavaMethod", "(Ljava/lang/String;)V");
        if (!jmid) {
                DBG_ERR("Cannot find java method... Terminating\n");
                return NULL;
        }

while(run) {
                struct timespec ts = {.tv_sec = 1, .tv_nsec = 0 };
                nanosleep(&ts, NULL);
                DBG_INFO("Trying to call method\n");
                callVoidMethodString(env, jCls, jmid, "***** Native2Java call works! \n");
        }
        (*jvm)->DetachCurrentThread(jvm);
        return NULL;
}

JNIEXPORT void JNICALL
Java_com_example_testservice_TestNative_startAthread( JNIEnv* env,
                jobject thiz)
{
        DBG_INFO("enter startAthread()\n");
        if (JNI_OK != (*env)->GetJavaVM(env, &jvm)) {
                DBG_ERR("Cannot access Java VM! Terminating call.\n");
                return;
        }
        DBG_INFO("Caching class tc...\n");
        /* Updated: jObj replaced with jCls */
        jCls = thiz;
        jobject globalRef = (*env)->NewGlobalRef(env, jCls);
        (*env)->DeleteLocalRef(env, jCls);
        jCls = globalRef;
        if (NULL == jCls) {
                DBG_ERR("Cannot cache class TronNative!\n");
                return;
        }
        /* UPD3: removed block below */
        /* Added see UPD2 */
        /*DBG_INFO("Caching class TestNative...\n");
         *jclass clazz = (*env)->FindClass(env, "com/example/testservice/TestNative");
         *if ((*env)->ExceptionCheck(env) == JNI_TRUE){
         *   (*env)->ExceptionDescribe(env);
         *   DBG_ERR("Exception while looking for TestNative class.\n");
         *   return;
         *}
         *jCls = (jclass)(*env)->NewGlobalRef(env, clazz);
         *
         *if ((*env)->ExceptionCheck(env) == JNI_TRUE){
         *   (*env)->ExceptionDescribe(env);
         *   DBG_ERR("Exception while trying to globalize TestNative class.\n");
         *   return;
         *}
         *(*env)->DeleteLocalRef(env, clazz);
         */
        /*****************/
        if (pthread_create(&t, NULL, thread_func, NULL)) {
                DBG_ERR("Cannot create thread!\n");
        }
}

static unsigned call_count = 0;

/* jobject -> jclass */
void callVoidMethodString(JNIEnv *env, jclass jcl, jmethodID jmid, const char *str) {
        jstring jstr = (*env)->NewStringUTF(env, str);
        char calls_str[50] = {0};
        sprintf(calls_str, "calls:%u\n", call_count++);
        (*env)->CallVoidMethod(env, jcl, jmid, jstr);
        if ((*env)->ExceptionCheck(env)) {
                DBG_ERR("There is some exceptional situation!\n");
                (*env)->ExceptionDescribe(env);
                (*env)->ExceptionClear(env);
        }
        (*env)->DeleteLocalRef(env, jstr);
        DBG_INFO(calls_str);
}

6. Therefore, the java method “public void stringJavaMethod(final String regStr)” is not called when calling CallVoidMethod() in native code. There are no errors and no method calls….

The log output I get when I start the test service:

11-12 14:05:05.396: I/GAV2(18672): Thread[GAThread,5,main]: No campaign data found.
11-12 14:05:05.586: I/Autostart(16419): Starting service...
11-12 14:05:05.586: D/dalvikvm(18934): Late-enabling CheckJNI
11-12 14:05:05.586: I/ActivityManager(441): Start proc com.example.testservice for service com.example.testservice/. TestService: pid=18934 uid=10097 gids={50097, 3003, 1028}
11-12 14:05:05.606: D/dalvikvm(18934): Debugger has detached; object registry had 1 entries
11-12 14:05:05.696: D/dalvikvm(441): GC_EXPLICIT freed 1485K, 39% free 16961K/27776K, paused 3ms+7ms, total 89ms
11-12 14:05:05.736: I/TestService(18934): onCreate
11-12 14:05:05.736: D/TestService(18934): Starting test controller
11-12 14:05:05.736: D/TestController(18934): Starting test native
11-12 14:05:05.736: D/dalvikvm(18934): No JNI_OnLoad found in /system/lib/liblog.so 0x420e0a08, skipping init
11-12 14:05:05.736: D/dalvikvm(18934): Trying to load lib /data/app-lib/com.example.testservice-1/liblayer-jni.so 0x420e0a08
11-12 14:05:05.736: D/dalvikvm(18934): Added shared lib /data/app-lib/com.example.testservice-1/liblayer-jni.so 0x420e0a08
11-12 14:05:05.736: I/JNITestService(18934): enter startAthread()
11-12 14:05:05.736: I/JNITestService(18934): Caching class tc...
11-12 14:05:05.736: D/TestService(18934): Test controller has started
11-12 14:05:06.736: I/JNITestService(18934): Trying to call method
11-12 14:05:06.736: I/JNITestService(18934): calls:0
11-12 14:05:07.736: I/JNITestService(18934): Trying to call method
11-12 14:05:07.736: I/JNITestService(18934): calls:1
... etc

Therefore, there are no messages from Java code

about calls from native code, and there are no Java method calls. That’s the problem. As a result, I plan to see a string in the log “Native2Java call works!” \n” is passed as a parameter to the JNI call.

Android.mk list :

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
ANDROID_NDK := /home/michael/Downloads/android-ndk-r8e
LOCAL_MODULE    :=         layer-jni
LOCAL_SRC_FILES :=         layer-jni.c
LOCAL_LDLIBS := -llog
TARGET_ARCH_ABI := armeabi-v7a
include $(BUILD_SHARED_LIBRARY)

Application.mk:

APP_ABI := armeabi-v7a
APP_PLATFORM := android-9
#APP_CPPFLAGS := -D__GXX_EXPERIMENTAL_CXX0X__ -std=gnu++11
STLPORT_FORCE_REBUILD := true
APP_STL := stlport_shared

AndroidManifest.xml:

<pre class=”lang-xml prettyprint-override”><manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.testservice"
android:versionCode="1"
android:versionName="1.0" >

<uses-sdk
android:minSdkVersion="14"
android:targetSdkVersion="17" />

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"></uses-permission>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.RECORD_AUDIO"/>
<uses-permission android:name="android.permission.USE_SIP"/>

<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name">
<service android:name=". TestService" android:exported="true">
<intent-filter>
<action android:name="com.example.testservice.TestService">
</action>
</intent-filter>
</service>
<receiver android:name=".autostart">
<intent-filter>
<action
android:name="android.intent.action.BOOT_COMPLETED">
</action>
</intent-filter>
</receiver>
</application>
</manifest>

If someone could help me with suggestions or ideas, I would really appreciate that I had to check/fix my logic.

UPD: If I change the Java method to static, GetMethodID() to GetStaticMethodID() and CallVoidMethod() to CallStaticVoidMethod() it starts working:

11-12 17:44:27.406: D/TestNative(21444): IT WORKS?***** Native2Java call works! *****
11-12 17:44:27.406: I/JNITestService(21444): calls:38
11-12 17:44:28.406: I/JNITestService(21444): Trying to call method
11-12 17:44:28.406: D/TestNative(21444): IT WORKS?***** Native2Java call works! *****
11-12 17:44:28.406: I/JNITestService(21444): calls:39
11-12 17:44:29.426: I/JNITestService(21444): Trying to call method

Still don’t know why non-static members don’t work….

UPD2: Fixed call GetMethodID() – replacing jObj with jCls. If jCls is obtained via FindClass(), the result remains the same (no calls, no errors). If jCls is obtained via GetObjectClass() instead of FindClass() — then an exception occurs while trying to get the method id using this new jCls:

11-12 18:17:59.926: D/TestService(22540): Test controller has started
11-12 18:17:59.926: E/JNITestService(22540): Cannot find java method... Terminating
11-12 18:17:59.926: W/dalvikvm(22540): threadid=12: thread exiting with uncaught exception (group=0x4198b700)
11-12 18:17:59.926: E/AndroidRuntime(22540): FATAL EXCEPTION: Thread-6532
11-12 18:17:59.926: E/AndroidRuntime(22540): java.lang.NoSuchMethodError: no method with name='stringJavaMethod' signature='(Ljava/lang/String;)V' in class Ljava/lang/Class;
11-12 18:17:59.926: E/AndroidRuntime(22540):    at dalvik.system.NativeStart.run(Native Method)

UPD3: Removed jObj, now only jCls are used. And fixed the startAthread parameter. But still no error, no call (non-static method, static version of the method is running).

Solution

jmethodID jmid = (*env)->GetMethodID(env, jObj, "stringJavaMethod", "(Ljava/lang/String;)V");

This usage of GetMethodID is incorrect. The second parameter should be the jclass type, not the jobject you are using.

To convert jobject to jclass, use

jclass GetObjectClass(JNIEnv *env, jobject obj);

Also, you should use GetStaticMethodID because your target method is static

Edit 1:

Check that the local method signature for public static native void startAthread() is incorrect. It should be

JNIEXPORT void JNICALL Java_Test_startAthread(JNIEnv *, jclass);

For now

JNIEXPORT void JNICALL Java_Test_startAthread(JNIEnv *, jobject);

This difference explains why it works when you change the corresponding Java function to static, because you actually have a jclass object instead of a jobject all the time.

Edit 2

Now that the function declaration is correct, the problem is clear: you are trying to call an instance method without an instance of the class. You need to change the static startAthread function to a non-static function and save the instance object, or you need to change the called Java method to a static function. According to your actual implementation, choose the implementation method that suits you.

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