一尘不染

在Android中从C ++调用Java方法

java android c++

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messageMegetJniString*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!");
}

干净的编译应用停止并显示下一条消息后:

ERROR/AndroidRuntime(742): FATAL EXCEPTION: main
        java.lang.NoSuchMethodError: messageMe
        at *.android.t3d.MainActivity.getJniString(Native Method)
        at *.android.t3d.MainActivity.onCreate(MainActivity.java:22)

显然,这意味着方法名称错误,但对我来说似乎不错。


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2020-03-21

共1个答案

一尘不染

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);

你正在做什么相当于jstr.messageMe()

由于你的方法无效,因此应调用:

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

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

2020-03-21