android jni 调用

约定不等于承诺〃 2022-08-04 00:42 193阅读 0赞

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