android JNI是连接android Java部分和C/C++部分的纽带，完整使用JNI需要Java代码和C/C++代码。其中C/C++代码用于生成库文件，Java代码用于引用C /C++库文件以及调用C/C++方法。
android Java部分代码：
/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.example.android.simplejni;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class SimpleJNI extends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView tv = new TextView(this);
// int sum = Native.add(2, 3);
int minus = Native.minus(10, 3);
tv.setText("10-3 = " + Integer.toString(minus));
setContentView(tv);
}
}
class Native {
static {
// The runtime will add "lib" on the front and ".o" on the end of
// the name supplied to loadLibrary.
System.loadLibrary("simplejni");
}
static native int add(int a, int b);
static native int minus(int a, int b);
}
Java代码说明：
1）SimpleJNI是一个activity的类对象，在该类对象中生成调用JNI函数的类对象，同时调用JNI方法，最后将JNI方法的结果显示到标题栏上；
2）SimpleJNI是一个引用和声明JNI库和函数的类，其中System.loadLibrary();函数用来引用JNI库，默认JNI库放在 android系统的/system/lib/目录下； static native int add(int a, int b);为声明需要在java程序中使用的JNI库中的函数；
JNI中java部分的代码到此就结束了，总结一下在java代码中需要做两件事：
1）使用System.loadLibrary()函数来引用JNI库；
2）声明调用JNI库的函数且前面添加native关键字；
android C/C++部分代码：
/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#define LOG_TAG "simplejni native.cpp"
#include <utils/Log.h>
#include <stdio.h>
#include "jni.h"
static jint
add(JNIEnv *env, jobject thiz, jint a, jint b) {
int result = a + b;
ALOGI("%d + %d = %d", a, b, result);
return result;
}
static jint
minus(JNIEnv *env, jobject thiz, jint a, jint b) {
int result = a - b;
ALOGI("%d - %d = %d", a, b, result);
return result;
}
static const char *classPathName = "com/example/android/simplejni/Native";
static JNINativeMethod methods[] = {
{"add", "(II)I", (void*)add },
{"minus", "(II)I", (void*)minus }
};
/*
* Register several native methods for one class.
*/
static int registerNativeMethods(JNIEnv* env, const char* className,
JNINativeMethod* gMethods, int numMethods)
{
jclass clazz;
clazz = env->FindClass(className);
if (clazz == NULL) {
ALOGE("Native registration unable to find class ‘%s‘", className);
return JNI_FALSE;
}
if (env->RegisterNatives(clazz, gMethods, numMethods) < 0) {
ALOGE("RegisterNatives failed for ‘%s‘", className);
return JNI_FALSE;
}
return JNI_TRUE;
}
/*
* Register native methods for all classes we know about.
*
* returns JNI_TRUE on success.
*/
static int registerNatives(JNIEnv* env)
{
if (!registerNativeMethods(env, classPathName,
methods, sizeof(methods) / sizeof(methods[0]))) {
return JNI_FALSE;
}
return JNI_TRUE;
}
// ----------------------------------------------------------------------------
/*
* This is called by the VM when the shared library is first loaded.
*/
typedef union {
JNIEnv* env;
void* venv;
} UnionJNIEnvToVoid;
jint JNI_OnLoad(JavaVM* vm, void* reserved)
{
UnionJNIEnvToVoid uenv;
uenv.venv = NULL;
jint result = -1;
JNIEnv* env = NULL;
ALOGI("JNI_OnLoad");
if (vm->GetEnv(&uenv.venv, JNI_VERSION_1_4) != JNI_OK) {
ALOGE("ERROR: GetEnv failed");
goto bail;
}
env = uenv.env;
if (registerNatives(env) != JNI_TRUE) {
ALOGE("ERROR: registerNatives failed");
goto bail;
}
result = JNI_VERSION_1_4;
bail:
return result;
}
JNI C/C++代码说明：
1）JNI_OnLoad()函数。该函数在Java程序调用System.loadLibrary()时，被调用执行，用于向JavaVM注册JNI函数等。在本例中首先通过参数JavaVM（Java虚拟机指针）获取当前应用程序所在的线程，即：JNIEnv。再通过调用 android::AndroidRuntime::registerNativeMethods()注册native实现的函数指针。
2）JNI函数和Java调用函数的映射关系。使用JNINativeMethod将java调用的函数名与JNI实现的函数名联系在一起；
3）JNI函数实现；
Android.mk代码：
#
# Copyright (C) 2008 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
# This makefile shows how to build a shared library and an activity that
# bundles the shared library and calls it using JNI.
TOP_LOCAL_PATH:= $(call my-dir)
# Build activity
LOCAL_PATH:= $(TOP_LOCAL_PATH)
include $(CLEAR_VARS)
LOCAL_MODULE_TAGS := samples
LOCAL_SRC_FILES := $(call all-subdir-java-files)
LOCAL_PACKAGE_NAME := SimpleJNI
LOCAL_JNI_SHARED_LIBRARIES := libsimplejni
LOCAL_PROGUARD_ENABLED := disabled
LOCAL_SDK_VERSION := current
include $(BUILD_PACKAGE)
# ============================================================
# Also build all of the sub-targets under this one: the shared library.
include $(call all-makefiles-under,$(LOCAL_PATH))
需要注意的是：
1）JNI C/C++部分的代码需要在android源代码树上进行编译，编译完成后我的做法是直接将生成的.so通过adb push方法上传到android虚拟机的/system/lib/目录下；
2）java代码可以在eclipse下直接编译且在虚拟机上执行；
编译JNI C/C++部分代码（在android内核源代码根目录下）：
#make XXXX
之后在out/target/product/generic/system/lib/目录下生成XXX.so
需要源码的同学 ：http://download.csdn.net/detail/tianyeming/9000955
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