NDK learning notes (I)

1.java The relationship between method and native instance method parameters .

java The instance method corresponds to jobject,java Static method correspondence jclass.

 public static native String stringFromJNI();
 
extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_testndk_JniUtil_stringFromJNI(JNIEnv *env, jclass thiz){
    }

public native String stringFromJNI();

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_testndk_JniUtil_stringFromJNI(JNIEnv *env, jobject thiz) {
    }

2.java And c/c++ Basic data type relationships for

3.java The relationship between application type and native

4. String related operations

hold java The string is converted to C character string

    //JNI_TRUE  The returned string address points to the copy 
    //JNI_FALSE  The returned string address points to a fixed object in the heap 
    jboolean bo = JNI_FALSE;

    // Create string 
    jstring ss = env->NewStringUTF("hello world");
    // take jstring Convert to an operable string 
    const char *str = env->GetStringUTFChars(ss, &bo);
    if (str != 0) {
    

        printf("111111111111111%s", str);
    }

Release string （ Unlike java There's a garbage collector , Application requires release ）

env->ReleaseStringUTFChars(ss,str);

5. Array operation

（1） Create array

New<Type>Array
Type: Corresponding Int、Char、Bollean etc. 

Using examples ：

    jintArray array = env->NewIntArray(10);

（2） Access array （ copy ）

        // Convert array 
        jint nativeARRAY[5];
        env->GetIntArrayRegion(array, 0, 5, nativeARRAY);
        // Operation array , from C Array orientation java Array commit changes 
        nativeARRAY[0] = 100;
        env->SetIntArrayRegion(array, 0, 5, nativeARRAY);

java

        int[] array = {
    10, 11, 12, 13, 14};
        arrayOperation(array);
        for (int i = 0; i < 5; i++) {
    

            Log.i("mainActivity", array[i] + "");
        }

Output results

2020-03-14 17:35:12.173 18150-18150/com.example.testndk I/mainActivity: 100
2020-03-14 17:35:12.173 18150-18150/com.example.testndk I/mainActivity: 11
2020-03-14 17:35:12.173 18150-18150/com.example.testndk I/mainActivity: 12
2020-03-14 17:35:12.173 18150-18150/com.example.testndk I/mainActivity: 13
2020-03-14 17:35:12.175 18150-18150/com.example.testndk I/mainActivity: 14

（3） Access array （ Operation on direct pointer ）

        // Get a direct pointer 
        jint *myInt = env->GetIntArrayElements(array, JNI_FALSE);
        // Operation array , Commit changes and free memory 
        myInt[0] = 1555;
        env->ReleaseIntArrayElements(array, myInt, 0);

java

        int[] array = {
    10, 11, 12, 13, 14};
        arrayOperation(array);
        for (int i = 0; i < 5; i++) {
    

            Log.i("mainActivity", array[i] + "");
        }

Output results

2020-03-14 20:45:50.064 19088-19088/com.example.testndk I/mainActivity: 1555
2020-03-14 20:45:50.065 19088-19088/com.example.testndk I/mainActivity: 11
2020-03-14 20:45:50.065 19088-19088/com.example.testndk I/mainActivity: 12
2020-03-14 20:45:50.065 19088-19088/com.example.testndk I/mainActivity: 13
2020-03-14 20:45:50.065 19088-19088/com.example.testndk I/mainActivity: 14

ReleaseIntArrayElements The third parameter mode explain .

6.NIO operation

    // Create a direct byte buffer 
    unsigned char *buffer = (unsigned char *) malloc(1024);
    jobject objBuffer = env->NewDirectByteBuffer(buffer, 1024);
    // Get buffer 
    unsigned char *buffer2 = (unsigned char *)env->GetDirectBufferAddress(objBuffer);

7. access domains

extern "C" JNIEXPORT void JNICALL
Java_com_example_testndk_MainActivity_fieldOperation(
        JNIEnv *env,
        jobject /* this */, jobject obj) {
    

    // Instance domain 
    // Get domain id,Ljava/lang/String;
    jclass cls = env->GetObjectClass(obj);
    jfieldID fId = env->GetFieldID(cls, "size", "Ljava/lang/String;");
    // Get domain 
    jstring str = (jstring) env->GetObjectField(obj, fId);
    const char *c = env->GetStringUTFChars(str, JNI_FALSE);
    LOGI("%s", c);

    // Static domain 
    // Get domain id
    jfieldID fId2 = env->GetStaticFieldID(cls, "staticSize", "Ljava/lang/String;");
    // Get domain 
    jstring str2 = (jstring) env->GetStaticObjectField(cls, fId2);
    const char *c2 = env->GetStringUTFChars(str2, JNI_FALSE);
    LOGI("%s", c2);
}

java

public class MyBox {
    

    private String size = "instance";
    private static String staticSize = "static";
}

        MyBox myBox = new MyBox();
        fieldOperation(myBox);

Output results

2020-03-14 23:53:00.647 21624-21624/com.example.testndk I/test-jni: instance
2020-03-14 23:53:00.647 21624-21624/com.example.testndk I/test-jni: static

8. Calling method

extern "C" JNIEXPORT void JNICALL
Java_com_example_testndk_MainActivity_methodOperation(
        JNIEnv *env,
        jobject /* this */, jobject obj) {
    
    // obtain class
    jclass cls = env->GetObjectClass(obj);
    // Get instance method id
    jmethodID mId = env->GetMethodID(cls, "instanceMethod", "()Ljava/lang/String;");
    // Invoking an instance method 
    jstring result = (jstring) env->CallObjectMethod(obj, mId);
    const char *c = env->GetStringUTFChars(result, JNI_FALSE);
    LOGI("%s",c);


    // Get static methods id
    jmethodID mId2 = env->GetStaticMethodID(cls, "staticMethod", "()Ljava/lang/String;");
    // Call static methods 
    jstring result2 = (jstring) env->CallStaticObjectMethod(cls, mId2);
    const char *c2 = env->GetStringUTFChars(result2, JNI_FALSE);
    LOGI("%s",c2);
}

java

public class MyBox {
    


    private String instanceMethod() {
    

        return "instance";
   }

    private static String staticMethod() {
    

        return "static";
    }
}

        MyBox myBox = new MyBox();
        methodOperation(myBox);

Output content

2020-03-15 13:50:54.937 21991-21991/com.example.testndk I/test-jni: instance
2020-03-15 13:50:54.937 21991-21991/com.example.testndk I/test-jni: static

9. Domain and method descriptors

 This type of data is a descriptor 
()Ljava/lang/String;
Ljava/lang/String;

android studio Descriptor configuration for , Makes it easier to get descriptors .
file→setting→tools→external tools
Click on + Number ,name at will , Others are as follows ：

program：$JDKPath$/bin/javap
arguments：-s -p $FileClass$
working directory：$ModuleFileDir$\build\intermediates\javac\debug\classes

Click on ok, complete .
Use ： Click menu tools→external tools→ Your name
effect ：

D:\Android_develop\android_studio\jre/bin/javap -s -p com.example.testndk.MyBox
Compiled from "MyBox.java"
public class com.example.testndk.MyBox {
    
  public com.example.testndk.MyBox();
    descriptor: ()V

  private java.lang.String instanceMethod();
    descriptor: ()Ljava/lang/String;

  private static java.lang.String staticMethod();
    descriptor: ()Ljava/lang/String;
}

Process finished with exit code 0

10. Use in native code log

#include <android/log.h>

#define LOG "test-jni" //  This is a custom LOG The logo of 
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG,LOG,__VA_ARGS__) //  Definition LOGD type 
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO,LOG,__VA_ARGS__) //  Definition LOGI type 
#define LOGW(...) __android_log_print(ANDROID_LOG_WARN,LOG,__VA_ARGS__) //  Definition LOGW type 
#define LOGE(...) __android_log_print(ANDROID_LOG_ERROR,LOG,__VA_ARGS__) //  Definition LOGE type 
#define LOGF(...) __android_log_print(ANDROID_LOG_FATAL,LOG,__VA_ARGS__) //  Definition LOGF type 

Use

    LOGI("%s",c);