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。
messageMe
getJniString*
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。
如果是对象方法,则需要将该对象传递给CallObjectMethod:
CallObjectMethod
jobject result = env->CallObjectMethod(obj, messageMe, jstr);
你正在做什么相当于jstr.messageMe()。
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。
()V
void