在Android中从C ++调用Java方法

Question

我试图从C ++获得一个简单的Java方法调用，而Java调用本机方法。这是Java代码：public class MainActivity extends Activity {    private static String LIB_NAME = "name";    static {        System.loadLibrary(LIB_NAME);    }    /** Called when the activity is first created. */    @Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        TextView tv = (TextView) findViewById(R.id.textview);        tv.setText(this.getJniString());    }    public void messageMe(String text) {        System.out.println(text);    }    public native String getJniString();}我试图messageMe在getJniString*从Java到本机的方法调用过程中从本机代码调用方法。native.cpp：#include <string.h>#include <stdio.h>#include <jni.h>jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){//    JavaVM *vm;//    JNIEnv *env;//    JavaVMInitArgs vm_args;//    vm_args.version = JNI_VERSION_1_2;//    vm_args.nOptions = 0;//    vm_args.ignoreUnrecognized = 1;////    // Construct a VM//    jint res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);    // Construct a String    jstring jstr = env->NewStringUTF("This string comes from JNI");    // First get the class that contains the method you need to call    jclass clazz = env->FindClass("the/package/MainActivity");    // Get the method that you want to call    jmethodID messageMe = env->GetMethodID(clazz, "messageMe", "(Ljava/lang/String;)V");    // Call the method on the object    jobject result = env->CallObjectMethod(jstr, messageMe);    // Get a C-style string    const char* str = env->GetStringUTFChars((jstring) result, NULL);    printf("%s\n", str);        // Clean up    env->ReleaseStringUTFChars(jstr, str);//    // Shutdown the VM.//    vm->DestroyJavaVM();    return env->NewStringUTF("Hello from JNI!");}

Answer 1

如果是对象方法，则需要将该对象传递给CallObjectMethod：

jobject result = env->CallObjectMethod(obj, messageMe, jstr);

您正在做什么相当于jstr.messageMe()。

由于您的方法无效，因此您应该调用：

env->CallVoidMethod(obj, messageMe, jstr);

如果要返回结果，则需要更改JNI签名（()V表示void返回类型的方法）以及Java代码中的返回类型。