Java 中通过jni接口调用native code
在上上篇中已经介绍了如何编译native code 生成 so动态库，这里再介绍下Java中如何通过JNI接口来调用动态库中的native code方法。
通过此篇文章，我们可以认识和了解如何完整地在Android的java code中调用native code。
1 创建一个 android project, 名字叫Why
2 在工程Why中添加一个Java类，class名为Jni。这个类是一个JNI接口的Java类，文件名为Jni.java。
package com.yarin.android.Why;
public class Jni {
public native int getCInt();
public native String getCString();
}
3 将工程Why下的 "src\com\yarin\android\Why" 目录下的Jni.java文件copy到“Why\bin\classes”下.
4 Generate Jni.class file via the command below:
javac jni.java
Then copy the Jni.class file generated to cover and replace the original Jni.class file in directory “Why\bin\classes\com\yarin\android\Why”.
------The original Jni.class file, you must build the java project first to generate it.
5 Generate c style header file
javah –classpath C:\android-ndk-r6b\myproject\Why\bin\classes com.yarin.android.Why.Jni
com.yarin.android.Why is my package name appeared in Jni.java file.
com_yarin_android_Why_Jni.h is like below:
/* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class com_yarin_android_Why_Jni */
#ifndef _Included_com_yarin_android_Why_Jni
#define _Included_com_yarin_android_Why_Jni
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: com_yarin_android_Why_Jni
* Method: getCInt
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_com_yarin_android_Why_Jni_getCInt
(JNIEnv *env, jobject object); ---you need to supplement the parameter name yourself
/* * Class: com_yarin_android_Why_Jni * Method: getCString * Signature: ()Ljava/lang/String; */ JNIEXPORT jstring JNICALL Java_com_yarin_android_Why_Jni_getCString (JNIEnv *env, jobject object);
#ifdef __cplusplus } #endif #endif
6 Add com_yarin_android_Why_Jni.c file corresponding to the above c style header file, then add implemented code.
#include <stdio.h>
#include <stdlib.h>
#include "com_yarin_android_Why_Jni.h"
int add()
{
int x,y;
x = 111;
y = 22;
x += y;
return x;
}
JNIEXPORT jint JNICALL Java_com_yarin_android_Why_Jni_getCInt
(JNIEnv *env, jobject object)
{
return add();
}
JNIEXPORT jstring JNICALL Java_com_yarin_android_Why_Jni_getCString
(JNIEnv *env, jobject object)
{
(*env)->NewStringUTF(env, " Why is ok ^_^ ----->> ");
}
7 然后实现在工程Why下创建目录jni，并且copy com_yarin_android_Why_Jni.c /h 到jni目录下。
8 在"Why\jni"目录下编写Android.mk ，在"android-ndk-r6b\jni"下编写Application.mk , 然后在NDK环境下编译native code，生成动态库libWhy.so。这个你已经从上上篇中知道的……。
9 在java工程中加入调用native 动态库的代码：
package com.yarin.android.Why;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class WhyActivity extends Activity {
static
{
System.loadLibrary("Why");
}
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Jni jni = new Jni();
TextView view = new TextView(this);
view.setText(jni.getCString() + Integer.toString(jni.getCInt()));
setContentView(view);
}
}
10 编译Why工程，然后 run as Android Application, 你就能看到native code的调用结果了。
Tip:
Usage of the command javah.
javah -d <outputdir> -classpath <classpath> <fully_qualified_class>
where:
'outputdir' is the directory where to put the generated header file
'classpath' contains an absolute path to the directory containing your root package (as mentionned by Glen)
'fully_qualified_class' is the name of the class containing native methods without .class extension
-jni option is not required (set by default)