Android JNI开发一，HelloJni

容易忘记，记个笔记，个人理解，可能有误，求高手指点

本例使用NDK开发的HelloJNI实例

代码结构如下如所示：

HelloJni.java的代码如：

/* * Copyright (C) 2009 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.hellojni;import android.app.Activity;import android.os.Bundle;import android.widget.TextView;import android.widget.Toast;public class HelloJni extends Activity{private TextViewtext_yuan;private TextViewtext_1;/** Called when the activity is first created. */@Overridepublic void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.main);text_yuan = (TextView) findViewById(R.id.text_yuan);text_yuan.setText(stringFromJNI());text_1 = (TextView) findViewById(R.id.text_1);text_1.setText(helloTest());System.out.println(helloTest());Toast.makeText(this, helloTest(), Toast.LENGTH_LONG).show();}/* * A native method that is implemented by the * 'hello-jni' native library, which is packaged * with this application. */public native String stringFromJNI();/* * This is another native method declaration that is *not* * implemented by 'hello-jni'. This is simply to show that * you can declare as many native methods in your Java code * as you want, their implementation is searched in the * currently loaded native libraries only the first time * you call them. * Trying to call this function will result in a * java.lang.UnsatisfiedLinkError exception ! */public native String unimplementedStringFromJNI();public native String helloTest();/* * this is used to load the 'hello-jni' library on application * startup. The library has already been unpacked into * /data/data/com.example.hellojni/lib/libhello-jni.so at * installation time by the package manager. */static{/**** * 加载JNI库 * loadLibrary方法中的String参数 与LOCAL_MODULE后面参数相同hello-jni */System.loadLibrary("hello-jni");}}

android.mk代码如下：

# Copyright (C) 2009 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.#LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := hello-jniLOCAL_SRC_FILES := hello-jni.c \                   my_hello.cinclude $(BUILD_SHARED_LIBRARY)

my_hello.c的代码如下：

#include <string.h>#include <jni.h>jstringJava_com_example_hellojni_HelloJni_helloTest( JNIEnv* env,  jobject thiz ){return (*env)->NewStringUTF(env, "wo cao ni ma de JNI!");}

jstring 表示返回值
Java_com_example_hellojni_HelloJni_helloTest
写法是Java+Android工程的包名+Android工程的Activity名+方法名,点号用下划线表示，这个写法很严格。
包名：com_example_hellojni
Activity名：HelloJni
方法名：helloTest
JNIEnv* env, jobject thiz是Native方法自带的参数，可以用来转换一个数据类型。也就是说其实这个helloTest是没有形参的。

hello-jni.c代码如下：

/* * Copyright (C) 2009 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. * */#include <string.h>#include <jni.h>/* This is a trivial JNI example where we use a native method * to return a new VM String. See the corresponding Java source * file located at: * *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java */jstringJava_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,                                                  jobject thiz ){#if defined(__arm__)  #if defined(__ARM_ARCH_7A__)    #if defined(__ARM_NEON__)      #if defined(__ARM_PCS_VFP)        #define ABI "armeabi-v7a/NEON (hard-float)"      #else        #define ABI "armeabi-v7a/NEON"      #endif    #else      #if defined(__ARM_PCS_VFP)        #define ABI "armeabi-v7a (hard-float)"      #else        #define ABI "armeabi-v7a"      #endif    #endif  #else   #define ABI "armeabi"  #endif#elif defined(__i386__)   #define ABI "x86"#elif defined(__x86_64__)   #define ABI "x86_64"#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */   #define ABI "mips64"#elif defined(__mips__)   #define ABI "mips"#elif defined(__aarch64__)   #define ABI "arm64-v8a"#else   #define ABI "unknown"#endif    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");}

效果如下：