Calling a java method from c in Android

Question

I m trying to get a simple Java method call from C   while Java calls native method  Here s the Java code   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        I m trying to call messageMe method from native code in the process of getJniString  method call from Java to native   native cpp    include  lt string h gt   include  lt stdio h gt   include  lt jni h gt   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  amp vm   void     amp env   amp vm args           Construct a String     jstring jstr   env- gt NewStringUTF  This string comes from JNI           First get the class that contains the method you need to call     jclass clazz   env- gt FindClass  the package MainActivity           Get the method that you want to call     jmethodID messageMe   env- gt GetMethodID clazz   messageMe     Ljava lang String  V           Call the method on the object     jobject result   env- gt CallObjectMethod jstr  messageMe          Get a C-style string     const char  str   env- gt GetStringUTFChars  jstring  result  NULL       printf   s n   str              Clean up     env- gt ReleaseStringUTFChars jstr  str             Shutdown the VM        vm- gt DestroyJavaVM         return env- gt NewStringUTF  Hello from JNI         After clean compilation app stops with next message   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    Apparently it means that method name is wrong  but it looks OK to me

User · Accepted Answer

If it s an object method  you need to pass the object to CallObjectMethod   jobject result   env- gt CallObjectMethod obj  messageMe  jstr     What you were doing was the equivalent of jstr messageMe     Since your is a void method  you should call   env- gt CallVoidMethod obj  messageMe  jstr    If you want to return a result  you need to change your JNI signature  the   V means a method of void return type  and also the return type in your Java code

User · Answer

Solution posted by Denys S  in the question post   I quite messed it up with c to c   conversion  basically env variable stuff   but I got it working with the following code for C      include  lt string h gt   include  lt stdio h gt   include  lt jni h gt   jstring Java the package MainActivity getJniString  JNIEnv  env  jobject obj        jstring jstr     env - gt NewStringUTF env   This comes from jni         jclass clazz     env - gt FindClass env   com inceptix android t3d MainActivity        jmethodID messageMe     env - gt GetMethodID env  clazz   messageMe     Ljava lang String  Ljava lang String         jobject result     env - gt CallObjectMethod env  obj  messageMe  jstr        const char  str     env - gt GetStringUTFChars env  jstring  result  NULL      should be released but what a heck  it s a tutorial        printf   s n   str        return   env - gt NewStringUTF env  str       And next code for java methods       public class MainActivity extends Activity       private static String LIB NAME    thelib        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                   please  let me live even though I used this dark programming technique     public String messageMe String text            System out println text           return text             public native String getJniString

[java] Calling a java method from c++ in Android

Examples related to java

Examples related to c++

Examples related to android

Examples related to java-native-interface

Examples related to android-ndk