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Generally, our NDK programming interfaces are used to call C++ in the Java layer, but in fact, the C++ layer can also call Java functions. The implementation method is as follows:

1. Get the class name: jclass cls = env->FindClass

2. Get class method: jmethodID mid = env->GetMethodID

3. Get class member variable: fieldID fid=env->GetFieldID

4. Generate class object: job obj=env->NewObject (jobect can also be passed down from the Java layer)

5. Call class member method: env->CallXXXMethod (XXX is the return value type of Java method)

Here is an example:

First, there is Java code. First, a JniTest class is generated, which contains a method of sayingHelloFromJava. The goal we want to achieve is to assign values ​​(String str), two shaping values ​​(int index1, int index2), and a shaping array (int[] intArray), and then print these values ​​in Java.

public class JniTest extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { (savedInstanceState); setContentView(); JNI j = new JNI(); (); } public JniTest() { ("TEST","JniTest Constructor"); } public int sayHelloFromJava(String str, int index1, int index2, int[] intArray) { ("TEST", str + " But I am show in java"); ("TEST", "index1 = " + index1 + " index2 = " + index2 ); int javaIndex = 5; for(int i = 0; i < ; ++i) { ("TEST", "intArray[i] = " + intArray[i]); } return javaIndex; } }

public class JNI { static { ("myjni"); } public native void write(); }

Then the code in C++

JNIEXPORT void JNICALL Java_cc_androidos_jni_JNI_write (JNIEnv *env, jobject j) { LOGI("calltest"); jstring str = NULL; jclass clz = env->FindClass("cc/androidos/jni/JniTest"); //获取clz的构造函数并生成一个对象 jmethodID ctor = env->GetMethodID(clz, "<init>", "()V"); jobject obj = env->NewObject(clz, ctor); // 如果是数组类型，则在类型前加[,如整形数组int[] intArray,则对应类型为[I,整形数组String[] strArray对应为[Ljava/lang/String; jmethodID mid = env->GetMethodID(clz, "sayHelloFromJava", "(Ljava/lang/String;II[I)I"); if (mid) { LOGI("mid is get"); jstring str1 = env->NewStringUTF("I am Native"); jint index1 = 10; jint index2 = 12; //env->CallVoidMethod(obj, mid, str1, index1, index2); // 数组类型转换 testIntArray能不能不申请内存空间 jintArray testIntArray = env->NewIntArray(10); jint *test = new jint[10]; for(int i = 0; i < 10; ++i) { *(test+i) = i + 100; } env->SetIntArrayRegion(testIntArray, 0, 10, test); jint javaIndex = env->CallIntMethod(obj, mid, str1, index1, index2, testIntArray); LOGI("javaIndex = %d", javaIndex); delete[] test; test = NULL; } }

Through this example, we can basically understand how the C++ layer calls back Java functions. In addition, there is a small trick here. If you don't know what the type of your Java layer is in C++, you can write this type in the native method and then use the javah method to generate the .h file. Just check the corresponding type comments of the .h file to know the result. For example: We want to know how to write the corresponding types of String and shaping arrays. We add a public native void type(String str, int[] arrayInt) method to native

public class JNI { static { ("myjni"); } public native void write(); public native void type(String str, int[] arrayInt); }

Then generate the corresponding .h file:

/* * Class: cc_androidos_jni_JNI * Method: type * Signature: (Ljava/lang/String;[I)V */ JNIEXPORT void JNICALL Java_cc_androidos_jni_JNI_type (JNIEnv *, jobject, jstring, jintArray);

Let's pay attention to the "Signature: (Ljava/lang/String;[I)V" in the comment, where Ljava/lang/String;Ljava/lang/String; is the type of String (note that semicolons cannot be lost), and [I is the type corresponding to the shaping array.