JNI基础学习笔记--基本数据类型的传递与返回

基本类型和本地类型对比表：

Java类型	本地类型	说明
boolean	jboolean	无符号，8位
byte	jbyte	无符号，8位
char	jchar	无符号，16位
short	jshort	有符号，16位
int	jint	有符号，32位
long	jlong	有符号，64位
float	jfloat	32位
double	jdouble
void	void	N/A

从上表可以看jni的基本类型还是很容易记住的，所有数据类型都在头文件 jni.h中定义，需要详细了解可以查看这个头文件。

为熟练各个类型的使用，通过一个demo来加强记忆，demo很简单在android 定义各个类型的本地方法，传入相关类型的值，然后从本地方法中返回相应的数据类型值。

本地方法定义类 代码如下：

package com.aidesudi.testjni;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;


public class MainActivity extends AppCompatActivity {



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        boolean b = passBooleanReturnBoolean(true);
        byte b1 = passByteReturnByte((byte) 10);
        char c = passCharReturnChar('#');
        short s = passShortReturnShort((short) 100);
        int i = passIntReturnInt(1000);
        long l = passLongReturnLong(234234242L);
        float f = passFloatReturnFloat(10.0f);
        double d = passDoubleReturnDouble(11.11);


        // 在Java中打印输出
        Log.i("MainActivity","boolean: "+b);
        Log.i("MainActivity","byte: "+b1);
        Log.i("MainActivity","char: "+c);
        Log.i("MainActivity","short: "+s);
        Log.i("MainActivity","int: "+i);
        Log.i("MainActivity","long: "+l);
        Log.i("MainActivity","float: "+f);
        Log.i("MainActivity","double: "+d);

    }

    // 定义本地方法
    public native boolean passBooleanReturnBoolean(boolean b);
    public native byte passByteReturnByte(byte b);
    public native char passCharReturnChar(char c);
    public native short passShortReturnShort(short s);
    public native int passIntReturnInt(int i);
    public native long passLongReturnLong(long l);
    public native float passFloatReturnFloat(float f);
    public native double passDoubleReturnDouble(double d);

    static {
        System.loadLibrary("native-lib");
    }

}

本地C++文件中代码：

#include 
#include 
#include "mylog.h"
#include "../java/com_aidesudi_testjni_MainActivity.h"

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passBooleanReturnBoolean
 * Signature: (Z)Z
 */
JNIEXPORT jboolean JNICALL Java_com_aidesudi_testjni_MainActivity_passBooleanReturnBoolean
        (JNIEnv *, jobject, jboolean b){

    LOGI("%d in %d bytes", b, sizeof(b));

    return !b;

}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passByteReturnByte
 * Signature: (B)B
 */
JNIEXPORT jbyte JNICALL Java_com_aidesudi_testjni_MainActivity_passByteReturnByte
        (JNIEnv *, jobject, jbyte b){
    LOGI("%d in %d bytes", b, sizeof(b));
    return b+1;
}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passCharReturnChar
 * Signature: (C)C
 */
JNIEXPORT jchar JNICALL Java_com_aidesudi_testjni_MainActivity_passCharReturnChar
        (JNIEnv *, jobject, jchar c){

    LOGI("%d in %d bytes", c, sizeof(c));
    c = '*';
    return c;
}


/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passShortReturnShort
 * Signature: (S)S
 */
JNIEXPORT jshort JNICALL Java_com_aidesudi_testjni_MainActivity_passShortReturnShort
        (JNIEnv *, jobject, jshort s){

    LOGI("%d in %d bytes", s, sizeof(s));

    return s - 1;
}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passIntReturnInt
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_com_aidesudi_testjni_MainActivity_passIntReturnInt
        (JNIEnv *, jobject, jint i){

    LOGI("%d in %d bytes", i, sizeof(i));

    return i + 1;
}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passLongReturnLong
 * Signature: (J)J
 */
JNIEXPORT jlong JNICALL Java_com_aidesudi_testjni_MainActivity_passLongReturnLong
        (JNIEnv *, jobject, jlong l){
    LOGI("%d in %d bytes", l, sizeof(l));

    return l + 1;
}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passFloatReturnFloat
 * Signature: (F)F
 */
JNIEXPORT jfloat JNICALL Java_com_aidesudi_testjni_MainActivity_passFloatReturnFloat
        (JNIEnv *, jobject, jfloat f){
    LOGI("%d in %d bytes", f, sizeof(f));

    return f + 1.5;
}

/*
 * Class:     com_aidesudi_testjni_MainActivity
 * Method:    passDoubleReturnDouble
 * Signature: (D)D
 */
JNIEXPORT jdouble JNICALL Java_com_aidesudi_testjni_MainActivity_passDoubleReturnDouble
        (JNIEnv *, jobject, jdouble d){

    LOGI("%d in %d bytes", d, sizeof(d));

    return d + 1.5;
}

最后打印结果如下：