Summarize the knowledge points related to JNI and NDK.

Reprinted please indicate: http://blog.csdn.net/feather_wch/article/details/79599270

JNI and NDK Programming

Version: 2018/3/18-1 (11:36)

Brief Introduction to JNI and NDK

1. What is JNI? Effect?

2. Why do you need JNI?

3. What is NDK?

4. Advantages of NDK

5. How to integrate JNI function in AS3.0

6. The role of JNIEXPORT and JNICALL keywords in cpp code?

7. When Java Virtual Machine loads so library, how to find the corresponding hative method in Java layer? (Static registration)

8. What is the parameter jobject in the automatically generated JNI function?

9. What is the dynamic registration of JNI?

10. Steps for dynamic registration of JNI

public class JniUtils2 {
    static {
        System.loadLibrary("native-lib");
    }
    public native String getStringFromC();

    public native void dynamicFunction();

    public native void dynamicFunction2();
}

#include "com_hao_jniapp_JniUtils2.h"
#include "android/log.h"

extern "C" {
JNIEXPORT jstring JNICALL Java_com_hao_jniapp_JniUtils2_getStringFromC
        (JNIEnv *env, jobject jobject1) {
    return env->NewStringUTF("I am from JniUtils2 Of Native Method");
}

// 6. JNI function (concrete implementation of native method in Java layer)
static void jniDynamicLog(JNIEnv *evn, jobject obj){
    __android_log_print(ANDROID_LOG_INFO, "feather", "JNI's Log: hello Java! Im from JNI");
}
static void jniTest(JNIEnv *evn, jobject obj){
    __android_log_print(ANDROID_LOG_INFO, "feather", "JNI's Log: hello Java! Im from JNI's Test");
}

/**==========================
 * 7. JNINativeMethod Structures: Recording the correspondence between java's native methods and JNI functions
 *==========================*/
JNINativeMethod nativeMethod[] = {  {"dynamicFunction", //1. Method name of Java layer native method
                                            "()V",      //2. Descriptor of Java Layer native Method
                                            (void*)jniDynamicLog} //3. Pointer of JNI function
                    ,{"dynamicFunction2", "()V", (void*)jniTest}  //4. native/jni of another binding relationship
};

/**===========================================================
 * 1. JNI_OnLoad Called when `System.loadLibarary()` loads so libraries in Java
 * @param jvm *jvm For Java virtual machine instances, Java VM is a structure.
 *==========================================================*/
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved) {

    JNIEnv *env;
    //2. *jvm calls GetEnv() to get the JNIEnv variable
    if (jvm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {

        return -1;
    }
    __android_log_print(ANDROID_LOG_INFO, "feather", "JNI's Log: JNI_OnLoad...");
    //3. The JNIEnv structure points to a function table and gets the JniUtils object
    jclass clz = env->FindClass("com/hao/jniapp/JniUtils2");
    //4. Dynamic registration of Java-level native methods and JNI functions described by JniUtils objects and nativeMethod structures
    env->RegisterNatives(clz, nativeMethod, sizeof(nativeMethod)/sizeof(nativeMethod[0]));

    //5. return the JNI version currently in use
    return JNI_VERSION_1_4;
}

}

11. The Role of JNI Native Method

typedef struct {
    const char* name;//Name of Java Layer native Method
    const char* signature;//Descriptor of Java Layer native Method
    void*       fnPtr;//Pointer corresponding to JNI function
} JNINativeMethod;

12. The Role of JNIEnv Structures

//1. Get the ID of a variable in a Java object
jfieldID GetFieldID(jclass clazz, const char* name, const char* sig)
//2. Obtain variables with boolean data type according to the ID of the variable
jboolean GetBooleanField(jobject obj, jfieldID fieldID)
//3. Get the ID of the corresponding method in the Java object
jmethodID GetMethodID(jclass clazz, const char* name, const char* sig)
//4. Call the method in the corresponding object according to the method ID, and the method returns void.
CallVoidMethod(jobject obj, jmethodID methodID, ...)
//5. Call the method in the corresponding object according to the method ID, and the method returns boolean.
CallBooleanMethod(jobject obj, jmethodID methodID, ...)

13. The Role of JNI Data Types

14. What kinds of data types are there in JNI?

15. JNI's original data type

Java Type	Native Typ	Description
boolean	jboolean	unsigned 8 bits
byte	jbyte	signed 8 bits
char	jchar	unsigned 16 bits
short	jshort	signed 16 bits
int	jint	signed 32 bits
long	jlong	signed 64 bits
float	jfloat	32 bits
double	jdouble	64 bits
void	void	N/A

16. JNI Reference Type?

jobject                     (all Java objects)
|
|-- jclass                  (java.lang.Class objects)
|-- jstring                 (java.lang.String objects)
|-- jarray                  (array)
|     |--jobjectArray       (object arrays)
|     |--jbooleanArray      (boolean arrays)
|     |--jbyteArray         (byte arrays)
|     |--jcharArray         (char arrays)
|     |--jshortArray        (short arrays)
|     |--jintArray          (int arrays)
|     |--jlongArray         (long arrays)
|     |--jfloatArray        (float arrays)
|     |--jdoubleArray       (double arrays)
|
|--jthrowable

17. The role of method ID and variable ID in JNI?

//Variable ID
struct _jfieldID;
typedef struct _jfieldID *jfieldID;

//Method ID
struct _jmethodID;
typedef struct _jmethodID *jmethodID; /* method IDs */

18. The Role of Class Descriptors in JNI

19. Method Descriptor?

Method Descriptor	Java Language Type
"()Ljava/lang/String;"	String f();
"(ILjava/lang/Class;)J"	long f(int i, Class c);
"([B)V"	String(byte[] bytes);

20. Data Type Descriptor

Field Desciptor	Java Language Type
Z	boolean
B	byte
C	char
S	short
I	int
J	long
F	floa
D	double

21. Array descriptor

Descriptor	Java Langauage Type
"[[I"	int[][]
"[[[D"	double[][][]

Field Desciptor	Java Language Type
"Ljava/lang/String;"	String
"[Ljava/lang/Object;"	Object[]

22. Static method of calling Java in JNI function

//JniUtils2.Java
public class JniUtils2 {
    static {
        System.loadLibrary("native-lib");
    }
    ...
  //1. Static method
    public static void staticShowMsg(String msg){
        Log.i("feather", "I'm Java Static method: get JNI's msg =" + msg);
    }
}

//xxx.cpp
void callJavaMethod(JNIEnv *env, jobject thiz){
  //1. Get the Java class
    jclass clazz = env->FindClass("com/hao/jniapp/JniUtils2");
    if(clazz == NULL){
        printf("find class JniUtils2 error!");
        return;
    }
  //2. Get method ID
    jmethodID  id = env->GetStaticMethodID(clazz, "staticShowMsg", "(Ljava/lang/String;)V");
    if(id == NULL){
        printf("find method staticShowMsg error!");
    }
  //3. Call the static method and pass in parameters
    jstring msg = env->NewStringUTF("Hello Java! I'm JNI message");
    env->CallStaticVoidMethod(clazz, id, msg);
}

23. Non-static method of calling Java in JNI function

//java
public class JniUtils2 {
    static {
        System.loadLibrary("native-lib");
    }
    ...
    public void showMsg(String msg){
        Log.i("feather", "I'm Java method: get msg =" + msg + "from JNI");
    }
}

void callAnothorJavaMethod(JNIEnv *env, jobject thiz){
    jclass clazz = env->FindClass("com/hao/jniapp/JniUtils2");
    if(clazz == NULL){
        printf("find class JniUtils error!");
        return;
    }
  //1. Getting the ID of a non-static method
    jmethodID  id = env->GetMethodID(clazz, "showMsg", "(Ljava/lang/String;)V");
    if(id == NULL){
        printf("find method showMsg error!");
    }
    jstring msg = env->NewStringUTF("Hello Java! I'm JNI message");
  //2. The emphasis is on the need for this pointer (`jobject thiz') that will represent the JniUtils 2 object.
    env->CallVoidMethod(thiz, id, msg);
}

Reference books

1. JNI and NDK Development Tutorials for AS3.0
2. Android JNI Foundation
3. How to Print log to logcat in C/C++ Layer of JNI
4. JNI API Document