Android APK JNI sample (JAVA JNI)

一个小实例介绍 Android JNI 如何实现步骤，其中有个遇到的加载库失败的教训：

1.  编写JAVA代码，写明要调用的本地动态链接库的本地方法：

wnplib.java

public class wnplib{
    public native void SayHello(String name);

    static
    {
        System.loadLibrary("wnp");
    }

    public static void main(String [] argv)
    {
        wnplib wp = new wnplib();
        wp.SayHello("myName");
    }
}

2. 编译javac 成.class文件

     

javac wnplib.java

3.  使用javah 生成该类对应的C语言.h文件

     

javah -classpath . wnplib

4. 使用C/C++实现C的各函数

5. 在Android Source code 中编译代码生成.so库文件

     注意：此处需要注意，目前Android 有两种支持ARM 和 X86 两种，在不同的代码里编译的库有限制，是不是能弄个不依赖的？ NDK编译我还没有实验。

    当编译ARM 和 X86 不一致的.so会出现如下的错误：

   

08-14 16:24:58.572 I/ActivityManager(  240): Start proc jni.wnptest.wnpjnitest for activity jni.wnptest.wnpjnitest/.WnptestActivity: pid=22268 uid=10000 gids={}
08-14 16:24:58.592 E/jdwp    (22268): Failed sending reply to debugger: Broken pipe
08-14 16:24:58.592 D/dalvikvm(22268): Debugger has detached; object registry had 1 entries
08-14 16:24:58.602 D/WNP-TEST(22268): /vendor/lib:/system/lib:/system/lib/arm
08-14 16:24:58.612 D/houdini (22268): [22268] Loading library(version: 2.0.7.42789 RELEASE)... successfully.
08-14 16:24:58.612 D/houdini (22268): [22268] Open Native Library /system/lib/libwnp.so failed.
08-14 16:24:58.612 W/dalvikvm(22268): Exception Ljava/lang/UnsatisfiedLinkError; thrown while initializing Ljni/wnptest/wnpjnitest/WnptestActivity;
08-14 16:24:58.612 W/dalvikvm(22268): Class init failed in newInstance call (Ljni/wnptest/wnpjnitest/WnptestActivity;)
08-14 16:24:58.612 D/AndroidRuntime(22268): Shutting down VM
08-14 16:24:58.612 W/dalvikvm(22268): threadid=1: thread exiting with uncaught exception (group=0x4194e6f0)
08-14 16:24:58.612 I/Process (22268): Sending signal. PID: 22268 SIG: 9
08-14 16:24:58.612 E/AndroidRuntime(22268): FATAL EXCEPTION: main
08-14 16:24:58.612 E/AndroidRuntime(22268): java.lang.ExceptionInInitializerError
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.Class.newInstanceImpl(Native Method)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.Class.newInstance(Class.java:1319)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1872)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1984)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.ActivityThread.access$600(ActivityThread.java:124)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1148)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.os.Looper.loop(Looper.java:137)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at android.app.ActivityThread.main(ActivityThread.java:4436)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.reflect.Method.invokeNative(Native Method)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.reflect.Method.invoke(Method.java:511)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at dalvik.system.NativeStart.main(Native Method)
08-14 16:24:58.612 E/AndroidRuntime(22268): Caused by: java.lang.UnsatisfiedLinkError: Cannot load library: get_lib_extents[753]:   121 - /system/lib/libwnp.so is not a valid ELF object
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.Runtime.loadLibrary(Runtime.java:370)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at java.lang.System.loadLibrary(System.java:535)
08-14 16:24:58.612 E/AndroidRuntime(22268):     at jni.wnptest.wnpjnitest.WnptestActivity.(WnptestActivity.java:39)
08-14 16:24:58.612 E/AndroidRuntime(22268):     ... 15 more
08-14 16:24:58.612 W/ActivityManager(  240):   Force finishing activity jni.wnptest.wnpjnitest/.WnptestActivity

6. Android APK 测试代码：

    注意：在加载库的时候需要 去掉库前缀字符串"lib"， libwnp.so 加载的时候只需写wnp即可。

package jni.wnptest.wnpjnitest;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class WnptestActivity extends Activity {
    
    /*
     * Create a game room.
     */
    public native int wnpCreateRoom(int port);
    
    
    static
    {   
        Log.d("WNP-TEST", System.getProperty("java.library.path"));
        //System.out.println(System.getProperty("java.library.path")); 
        try {
            System.loadLibrary("wnp");
        } catch (UnsatisfiedLinkError ule) {
            System.err.println("WARNING: Could not load library!");
        }
    }
    
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

    }

}

7. 注意Android APK 加载库需要在JNI中指定JAVA测试的类名及路径

 *Java Application class path
 */
static const char *classPathName = "jni/wnptest/wnpjnitest/WnptestActivity";

static JNINativeMethod methods[] = {

  {"wnpSendDdata", "([BI)I", (void*)JNICALL Java_WNP_wnpSendDdata },

  {"wnpGetData", "()[B", (void*)JNICALL Java_WNP_wnpGetData },

  {"wnpCreateRoom", "(I)I", (void*)JNICALL Java_WNP_wnpCreateRoom },

  {"wnpJoinRoom", "(Ljava/lang/String;I)I", (void*)JNICALL Java_WNP_wnpJoinRoom },

  {"wnpQuitRoom", "()I", (void*)JNICALL Java_WNP_wnpQuitRoom },

};


/*
 * Register methods for one class.
 */
static int registerNativeMethods(JNIEnv* env, const char* className,
    JNINativeMethod* gMethods, int numMethods)
{
    jclass clazz;

    clazz = (*env)->FindClass(env, className);
    if (clazz == NULL)
        return JNI_FALSE;
    if ((*env)->RegisterNatives(env, clazz, gMethods, numMethods) < 0)
    {
        printf("register nativers error");
        return JNI_FALSE;
    }
    return JNI_TRUE;
}


/* 
 * Register methods for all classes. 
 * 
 * returns JNI_TRUE on success. 
 */
static int registerNatives(JNIEnv* env)
{

  if (!registerNativeMethods(env, classPathName,
                 methods, sizeof(methods) / sizeof(methods[0]))) {
    return JNI_FALSE;
  }
  return JNI_TRUE;

}

/* 
 * Called by the VM when the shared library is loaded. 
 */
jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
    JNIEnv* env = NULL;
    jint result = -1;

    WNP_LOGD("Entering JNI_OnLoad\n");

    if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_6) != JNI_OK)
        goto bail;

    assert(env != NULL);

    if (!registerNatives(env))
        goto bail;

    /* success -- return valid version number */
    result = JNI_VERSION_1_6;

bail:
    printf("Leaving JNI_OnLoad (result=0x%x)\n", result);
    return result;
}

8. 好的现在你可以测试实现的JNI了。