Android learning note _ JNI_c calls java code

1. Compile native METHOD (java2c) and non-native method (c2java ):

Package com. example. provider; public class CallbackJava {// C call the java empty method public void helloFromJava () {System. out. println ("hello from java");} // C calls the public int Add (int x, int y) method with two int parameters in java {int result = x + y; system. out. println ("java result =" + result); return result;} // C calls the public void printString (string s) {System. out. println ("java" + s);} // static method public static void printStaticStr (String s) {System. out. println ("java static->" + s) ;}// let the c code call the corresponding java code public native void callHelloFromJava (); public native void callAdd (); public native void callPrintString (); // call a static c code public native void callPrintStaticStr ();}

2. Generate a signature using the javah command and generate a native method signature using the javap command:

　　

3. c code

FindClass: Find the class file in java

GetMethodID: locate the method to be called based on the class and method signature.

# Include <stdio. h> # include <jni. h> # include <malloc. h> # include "com_example_provider_CallbackJava.h"; # include <android/log. h> # define LOG_TAG "System. out. c "# define LOGD (...) _ android_log_print (ANDROID_LOG_DEBUG, LOG_TAG, _ VA_ARGS _) # define LOGI (...) _ android_log_print (ANDROID_LOG_INFO, LOG_TAG, _ VA_ARGS _) jmethodID getMethodId (JNIEnv * env, char * methodname, char * signname) {jclass jclazz = (* env) -> FindClass (env, "com/example/provider/CallbackJava"); if (jclazz = 0) {LOGD ("class not find"); return ;} // 2. Find the method id in the class, the third parameter method name, and the fourth parameter. The jmethodID methodid = (* env)-> GetMethodID (env, jclazz, methodname, signname); if (methodid = 0) {LOGD ("method not find"); return;} return methodid;} JNIEXPORT void JNICALL encode (JNIEnv * env, jobject obj) {// 1. Find the class file in java // The second parameter is the class name (package name) jclass jclazz = (* env)-> FindClass (env, "com/example/provider/CallbackJava"); if (jclazz = 0) {LOGD ("class not find"); return ;} // 2. Find the method id in the class, the third parameter method name, and the fourth parameter. The jmethodID methodid = (* env)-> GetMethodID (env, jclazz, "helloFromJava", "() V"); if (methodid = 0) {LOGD ("helloFromJava method not find"); return ;} // 3. Call method // void (* CallVoidMethod) (JNIEnv *, jobject, jmethodID ,...); (* env)-> CallVoidMethod (env, obj, methodid);} JNIEXPORT void JNICALL inline (JNIEnv * env, jobject obj) {jmethodID mid = getMethodId (env, "Add ", "(II) I"); int result = (* env)-> CallIntMethod (env, obj, mid, 3, 6); LOGD ("c to java add: d % ", result);} JNIEXPORT void JNICALL encode (JNIEnv * env, jobject obj) {LOGD ("c to java callPrintString start"); jmethodID mid = getMethodId (env, "printString ", "(Ljava/lang/String;) V"); (* env)-> CallVoidMethod (env, obj, mid, (* env)-> NewStringUTF (env, "call printString finish"); LOGD ("c to java callPrintString finish");} JNIEXPORT void JNICALL handle (JNIEnv * env, jobject obj) {jclass jclazz = (* env) -> FindClass (env, "com/example/provider/CallbackJava"); if (jclazz = 0) {LOGD ("class not find"); return ;} // call GetStaticMethodID and CallStaticXXXMethod jmethodID methodid = (* env)-> GetStaticMethodID (env, jclazz, "printStaticStr", "(Ljava/lang/String;) V "); if (methodid = 0) {LOGD ("printStaticStr method not find"); return;} (* env)-> CallStaticVoidMethod (env, jclazz, methodid, (* env) -> NewStringUTF (env, "static hello java "));}