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。