使用NDK移植开源项目,JNI的使用技巧
fmms
12年前
<h2> jni 的介绍</h2> <p> <span style="line-height:25px;background-color:#ffffff;font-family:arial, 宋体, sans-serif;">JNI 是Java Native Interface的缩写，中文为JAVA本地调用。从Java1.1开始，Java Native Interface(JNI)标准成为java平台的一部分，它允许Java代码和其他语言写的代码进行交互。JNI一开始是为了本地已编译语言，尤其是 C和C++而设计的，但是它并不妨碍你使用其他语言，只要调用约定受支持就可以了。以下介绍Android 中如何使用jni移植开源库的技巧.</span></p> <h2> JNI日志输出到Logcat中</h2> #include <android/log.h> <br /> #define LOG_TAG "===xcloud===" <br /> #define LOGI(...) <u>android_log_print(ANDROID_LOG_INFO,LOG_TAG,</u>VA_ARGS__) <br /> #define LOGW(...) <u>android_log_print(ANDROID_LOG_WARN,LOG_TAG,</u>VA_ARGS__) <br /> #define LOGE(...) <u>android_log_print(ANDROID_LOG_ERROR,LOG_TAG,</u>VA_ARGS__) <br /> <p> </p> <p>Android.mk文件添加编译模块：<br /> LOCAL_LDLIBS=-lm -llog<br /> 使用方法：<br /> LOGE("%s",test);</p> <h2>JNI调用Java方法</h2> 以调用String 的getBytes方法为例： <br /> 第一步： <br /> jclass clsstring = (*env)->FindClass(env,"java/lang/String"); //找到Java String 类 <br /> 第二步： <br /> jstring strencode = (*env)->NewStringUTF(env,"utf-8"); //得到一个jstring 对象 <br /> 第三步： <br /> jmethodID mid = (*env)->GetMethodID(env,clsstring, "getBytes", "(Ljava/lang/String;)[B"); //得到getBytes方法 <br /> <p> </p> <p>第四步：<br /> jbyteArray barr= (jbyteArray)(*env)->CallObjectMethod(env,jstr, mid, strencode); //“jstr 为传入的字符串”调用getBytes方法</p> <p>第五步：<br /> jsize alen = (*env)->GetArrayLength(env,barr); //得到数组长度</p> <p> </p> <h2>jstring 转换 char*</h2> char* test=(char*)(*env)->GetStringUTFChars(env,jstringVariable,NULL); <br /> <p> </p> <p> </p> <h2>返回一个java对象数组</h2> 第一步： <br /> jclass objectClass=(*env)->FindClass(env,"com/xuzhitech/xcloud/resource"); //找到对应的java 类（对象） <br /> 第二步： <br /> jobjectArray array=(*env)->NewObjectArray(env,currentListCount,objectClass,NULL); //通过取到的java类(对象)创建一个指定固定大小的数组 <br /> 第三步： <br /> jfieldID _uri=(*env)->GetFieldID(env,objectClass,"uri","Ljava/lang/String;");//找到对象中的列 <br /> 注意：在JNI中并未提供jstring 类型的对象，所以必须通过L指定包名找到该类，在有提供的类型中，可以直接使用该类型的大写首字母(jlong 需使用J) <br /> 如jint 类型可以如此编写: <br /> jfieldID _vcr=(*env)->GetFieldID(env,objectClass,"is_vcr","I");对应的JNI提供类型可以参考如下 <a title="http://download.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html" href="/misc/goto?guid=4959518068407399825">http://download.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html</a> <br /> 第四步： <br /> jmethodID jid = (*env)->GetMethodID(env,objectClass,"<init>","()V");//"<init>"代表可以访问默认构造函数 <br /> jobject jobj=(*env)->NewObject(env,objectClass,jid); //通过第一步找到的jclass创建对应的对象 <br /> <p> </p> <p>第五步：<br /> (*env)->SetObjectField(env,jobj,_modtime,jmodtime); //为对象中的每一列赋值。注意：如果JNI有提供的数据类型，可按提供的类型为对象中的列赋值，<br /> 如：(*env)->SetIntField(env,jobj,_vcr,current->is_vcr);<br /> 第六步：<br /> (*env)->SetObjectArrayElement(env,array,i,jobj);将对象设置进第二步声明的数组中<br /> DeleteLocalRef(env,jobj);//删除引用对象</p> <h2>在JNI中循环读取对象数组</h2> <p> </p> <div class="cnblogs_code"> <div> <span style="color:#0000ff;">for</span> (i = <span style="color:#800080;">0</span> ;i < currentListCount ;i++) { <br /> jobject obj = (*env)->GetObjectArrayElement(env,array,i); <br /> jstring jstr=(*env)->GetObjectField(env,obj,_uri); <br /> LOGE( <span style="color:#800000;">"</span> <span style="color:#800000;">=====uri===%s====</span> <span style="color:#800000;">"</span>,jstringtoChar(env,jstr)); <br /> } <br /> 上面使用到的jstringtoChar方法代码： <br /> <span style="color:#008000;">/*</span> <span style="color:#008000;">*<br /> *jstring to char<br /> </span> <span style="color:#008000;">*/</span> <br /> <span style="color:#0000ff;">char</span>* jstringtoChar(JNIEnv* env,jstring jstr){ <br /> <span style="color:#0000ff;">char</span>* rtn = NULL; <br /> jclass clsstring = (*env)->FindClass(env, <span style="color:#800000;">"</span> <span style="color:#800000;">java/lang/String</span> <span style="color:#800000;">"</span>); <br /> jstring strencode = (*env)->NewStringUTF(env, <span style="color:#800000;">"</span> <span style="color:#800000;">utf-8</span> <span style="color:#800000;">"</span>); <br /> jmethodID mid = (*env)->GetMethodID(env,clsstring, <span style="color:#800000;">"</span> <span style="color:#800000;">getBytes</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)[B</span> <span style="color:#800000;">"</span>); <br /> jbyteArray barr= (jbyteArray)(*env)->CallObjectMethod(env,jstr, mid, strencode); <br /> jsize alen = (*env)->GetArrayLength(env,barr); <br /> jbyte* ba = (*env)->GetByteArrayElements(env,barr, JNI_FALSE); <br /> <span style="color:#0000ff;">if</span> (alen > <span style="color:#800080;">0</span>) <br /> { <br /> rtn = ( <span style="color:#0000ff;">char</span>*)malloc(alen + <span style="color:#800080;">1</span>); <br /> memcpy(rtn, ba, alen); <br /> rtn[alen] = <span style="color:#800080;">0</span>; <br /> } <br /> (*env)->ReleaseByteArrayElements(env,barr, ba, <span style="color:#800080;">0</span>); <br /> <span style="color:#0000ff;">return</span> rtn; </div> </div> <p>} </p> <p> </p> <h2>cpp JNI与 c JNI的主要注意事项：</h2> 用Ｃ编写jni文件，访问JNI内置提供方法格式: <br /> (*env)->SetObjectArrayElement(env,array,i,jobj); <br /> 用ＣＰＰ编写JNI文件，访问JNI内置提供方法格式： <br /> env->SetObjectArrayElement(array,i,jobj); <br /> <span style="color:red;">另外，使用CPP编写的JNI代码，在调用C语言编写的库的时候，要添加以下代码，才可以正常使用（不然在链接的时候找不到相关接口：undefined reference.....）：</span> <br /> <span style="color:red;">ifdef __cplusplus</span> <br /> <span style="color:red;">extern "C" {</span> <br /> <span style="color:red;">#endif</span> <br /> <span style="color:red;">... //引入的头文件</span> <br /> <span style="color:red;">#ifdef __cplusplus</span> <br /> <span style="color:red;">}</span> <br /> <span style="color:red;">#endif</span> <br /> <p> </p> <p>其他用法几乎一致。</p> <p> </p> <h2>JNI 使用Native注册</h2> 按照上面的方法写函数体，必须遵循JNI官方的一大堆标准进行方法的定义，有时候方法一多，不大好管理，也不利用查看，并且每次都要写一大堆恶心的标准方法名也不是一件好事。对此JNI有一套可以通过Native 注册的机制可以使用，以方便函数体的编写。 <br /> <p> </p> <p>以下是Android 调用JNI注册Natives 的步骤：<br /> 第一步<br /> 声明Java中要调用jni 的类路径：<br /> static const char *className="com/xuzhitech/xcloud/cadaver";<br /> 第二步<br /> 创建方法格式结构体：<br /> struct JNINativeMethod {<br /> const char* name;//method name <br /> const char* signature; //java method return value <br /> void* fnPtr;//c/c++ method <br /> } ;</p> <p>第三步<br /> 使用结构体注册需要供Android 调用的方法体：</p> <div class="cnblogs_code"> <div> <span style="color:#0000ff;">static</span> JNINativeMethod methods[] = { <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">StringTestOne</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()Ljava/lang/String;</span> <span style="color:#800000;">"</span>, ( <span style="color:#0000ff;">void</span>*)StringTestOne}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">executels</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()[Lcom/xuzhitech/xcloud/resource;</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)executels}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getProgress</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()Lcom/xuzhitech/xcloud/fileProgress;</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getProgress}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getdownloadState</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getdownloadState}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">changeExecutable</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)changeExecutable}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">Login</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)Login}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getCurrentListCount</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getCurrentListCount}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">mkcol</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)mkcol}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">deleteFile</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)deleteFile}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">deleteCol</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)deleteCol}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getFullUri</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()Ljava/lang/String;</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getFullUri}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">cdCommand</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)cdCommand}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">logout</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">()V</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)logout}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">doCopy</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)doCopy}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">doMove</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)doMove}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getFile</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;)V</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getFile}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">putFile</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;)V</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)putFile}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">lock</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*) <span style="color:#0000ff;">lock</span>}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">unlock</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)unlock}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">getToken</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)Ljava/lang/String;</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)getToken}, <br /> { <span style="color:#800000;">"</span> <span style="color:#800000;">isSetToken</span> <span style="color:#800000;">"</span>, <span style="color:#800000;">"</span> <span style="color:#800000;">(Ljava/lang/String;)I</span> <span style="color:#800000;">"</span>,( <span style="color:#0000ff;">void</span>*)isSetToken}, </div> </div> <p>}; </p> <p>第一个参数为：Java实现要调用的方法名称<br /> 第二个参数为：该方法需要的返回值与方法参数，如－>(方法参数)返回值，详细使用参考如下：<br /> 具体的每一个字符的对应关系如下</p> <p>字符 Java类型 C类型</p> <div class="cnblogs_code"> <div> V <span style="color:#0000ff;">void</span> <span style="color:#0000ff;">void</span> <br /> Z jboolean boolean <br /> I jint <span style="color:#0000ff;">int</span> <br /> J jlong <span style="color:#0000ff;">long</span> <br /> D jdouble <span style="color:#0000ff;">double</span> <br /> F jfloat <span style="color:#0000ff;">float</span> <br /> B jbyte <span style="color:#0000ff;">byte</span> <br /> C jchar <span style="color:#0000ff;">char</span> <br /> S jshort <span style="color:#0000ff;">short</span> </div> </div> <p> </p> <p>数组则以"["开始，用两个字符表示</p> <p></p> <div class="cnblogs_code"> <div> [I jintArray <span style="color:#0000ff;">int</span>[] <br /> [F jfloatArray <span style="color:#0000ff;">float</span>[] <br /> [B jbyteArray <span style="color:#0000ff;">byte</span>[] <br /> [C jcharArray <span style="color:#0000ff;">char</span>[] <br /> [S jshortArray <span style="color:#0000ff;">short</span>[] <br /> [D jdoubleArray <span style="color:#0000ff;">double</span>[] <br /> [J jlongArray <span style="color:#0000ff;">long</span>[] <br /> [Z jbooleanArray boolean[] </div> </div> 也可返回任意java对象，如上代码的()[Lcom/xuzhitech/xcloud/resource;代表一个不带参数的方法，并且返回java 类中的Lcom/xuzhitech/xcloud/resource;数组。'['代表数组的意思,'L'代表类的意思 <br /> <p> </p> <p>第三个参数为，第一个参数需要映射的本地c/c++对应的函数指针方法。</p> <p>第四步<br /> 在加载jni的时候指定JNI版本并且通过传入进来的class路径注册Natives 方法</p> <p> </p> <div class="cnblogs_code"> <div> <span style="line-height:21px;font-family:verdana, 'courier new';font-size:14px;">jint JNI_OnLoad(JavaVM* vm, </span> <span style="line-height:21px;font-family:verdana, 'courier new';color:#0000ff;font-size:14px;">void</span> <span style="line-height:21px;font-family:verdana, 'courier new';font-size:14px;">* reserved){</span> </div> <p>jint result = JNI_ERR;<br /> JNIEnv* env = NULL;<br /> jclass clazz;<br /> <span style="color:#0000ff;">int</span> methodsLenght;<br /> <span style="color:#0000ff;">if</span> ((*vm)->GetEnv(vm, (<span style="color:#0000ff;">void</span>**) &env, JNI_VERSION_1_4) != JNI_OK) {<br /> LOGE(<span style="color:#800000;">"</span><span style="color:#800000;">ERROR: GetEnv failed\n</span><span style="color:#800000;">"</span>);<br /> <span style="color:#0000ff;">return</span> JNI_ERR;<br /> }<br /> <span style="color:#008000;">//</span><span style="color:#008000;"> assert(env != NULL); </span><span style="color:#008000;"><br /> </span>clazz = (*env)->FindClass(env,className);<br /> <span style="color:#0000ff;">if</span> (clazz == NULL) {<br /> LOGE(<span style="color:#800000;">"</span><span style="color:#800000;">Native registration unable to find class '%s'</span><span style="color:#800000;">"</span>, className);<br /> <span style="color:#0000ff;">return</span> JNI_ERR;<br /> }<br /> methodsLenght = <span style="color:#0000ff;">sizeof</span>(methods) / <span style="color:#0000ff;">sizeof</span>(methods[<span style="color:#800080;">0</span>]);<br /> <span style="color:#0000ff;">if</span> ((*env)->RegisterNatives(env,clazz, methods, methodsLenght) < <span style="color:#800080;">0</span>) {<br /> LOGE(<span style="color:#800000;">"</span><span style="color:#800000;">RegisterNatives failed for '%s'</span><span style="color:#800000;">"</span>, className);<br /> <span style="color:#0000ff;">return</span> JNI_ERR;<br /> }<br /> <span style="color:#008000;">//</span><span style="color:#008000;"> </span><span style="color:#008000;"><br /> </span>result = JNI_VERSION_1_4;<br /> <span style="color:#0000ff;">return</span> result;<br /> }</p> </div> <p> </p> <p> </p> <p>注意，使用Natives注册运行程序时，它会先检测jni与java类使用jni 类的native 方法是否相对应与一致，即使你没有使用到该 方法也会进行检测。<br /> 第五步<br /> 通过上面的修改，c/c++编写的jni 文件就可以不带一大串标准名称了，您可以像正常编写c一样编写你的jni函数，如下：</p> <div class="cnblogs_code"> <div> <span style="color:#008000;">/*</span> <span style="color:#008000;"><br /> *move file<br /> </span> <span style="color:#008000;">*/</span> <br /> jint doMove(JNIEnv* env,jobject thiz,jstring jsrc,jstring jdest,jstring jrename){ <br /> <span style="color:#0000ff;">char</span>* src=( <span style="color:#0000ff;">char</span>*)(*env)->GetStringUTFChars(env,jsrc,NULL); <br /> <span style="color:#0000ff;">char</span>* dest=( <span style="color:#0000ff;">char</span>*)(*env)->GetStringUTFChars(env,jdest,NULL); <br /> <span style="color:#0000ff;">char</span>* rename=( <span style="color:#0000ff;">char</span>*)(*env)->GetStringUTFChars(env,jrename,NULL); <br /> <span style="color:#0000ff;">int</span> returnValue=multi_move(src,dest,rename); <br /> <span style="color:#008000;">//</span> <span style="color:#008000;"> free(src);</span> <span style="color:#008000;"><br /> </span>(*env)->ReleaseStringUTFChars(env,jsrc,src); <br /> <span style="color:#008000;">//</span> <span style="color:#008000;"> free(dest);</span> <span style="color:#008000;"><br /> </span>(*env)->ReleaseStringUTFChars(env,jdest,dest); <br /> <span style="color:#008000;">//</span> <span style="color:#008000;"> free(rename);</span> <span style="color:#008000;"><br /> </span>(*env)->ReleaseStringUTFChars(env,jrename,rename); <br /> <span style="color:#0000ff;">return</span> returnValue; <br /> } </div> </div> <p> </p> <p> 注:</p> <p>JNI的一些基本方法的使用都可以参考这个网站:http://docs.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html</p> <p> </p> <p>希望对你有帮助. <br /> <br /> 转自：<a href="/misc/goto?guid=4959518068491710849" target="_blank">http://www.cnblogs.com/TerryBlog/archive/2012/02/07/2340902.html</a></p>