JNI加载Native Library 以及 跨线程和Qt通信

Part1

Java Native Interface-JNI-JAVA本地调用

JNI标准是Java平台的一部分, 允许Java代码和其他语言进行交互;

开始实现->

Step 1) 编写Java代码, 编写一个JNI接口HelloJNI.java

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publicclass HelloJNI {

   static{

      System.loadLibrary("hello");// hello.dll (Windows) or libhello.so (Unixes)

   }

   // A native method that receives nothing and returns void

   privatenative void sayHello();

 

   publicstatic void main(String[] args) {

      newHelloJNI().sayHello();  // invoke the native method

   }

}

sayHello()是一个native方法, 这个方法会在生成的JNI header文件中声明C/C++的函数; 

loadLibrary()会在当前路径(实际上是Java Library Path)下寻找并加载名为hello的动态库, 所有的dependency都会在当前路径下加载; 
对于不同的平台loadLibrary()会自动搜索不同的后缀名; e.g. Sample.dll(Windows), libSample.so(Linux);
你也可以指定路径: "-Djava.library.path=path_to_lib", 路径错误的话会有"UnsatisfiedLinkError";

相应还有load()函数, 需要指定路径和dependency的路径; 
dlltool http://sourceware.org/binutils/docs/binutils/dlltool.html

Note 动态库加载必须在static块内, 保证首先进行;

 

Step 2) 编译和生成C/C++ header

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javac HelloJNI.java

编译Java生成class;

 

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javah HelloJNI

javah命令会生成相应的header: HelloJNI.h

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/* DO NOT EDIT THIS FILE - it is machine generated */

#include <jni.h>

/* Header for class HelloJNI */

 

#ifndef _Included_HelloJNI

#define _Included_HelloJNI

#ifdef __cplusplus

extern"C" {

#endif

/*

 * Class:     HelloJNI

 * Method:    sayHello

 * Signature: ()V

 */

JNIEXPORTvoid JNICALL Java_HelloJNI_sayHello

  (JNIEnv *, jobject);

 

#ifdef __cplusplus

}

#endif

#endif

在相应的cpp文件中实现函数: JNIEXPORT void JNICALL Java_HelloJNI_sayHello(JNIEnv *, jobject);

名字转换的格式: Java_{package_and_classname}_{function_name}(JNI arguments)

-JNIEnv*参数: 指向JNI的环境, 给你调用JNI函数的权限;

-jobject参数: 指向Java的"this"对象;

-extern "C"会被C++编译器识别, 把函数用C的命名方式来编译.

 

Step 3) 编译C/C++库

HelloJN.cpp的实现:

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#include <stdio.h>

#include "HelloJNI.h"

JNIEXPORTvoid JNICALL Java_HelloJNI_sayHello(JNIEnv *env, jobject thisObj) {

   printf("Hello World!\n");

   return;

}

>jni.h的路径一般是在<JAVA_HOME>\include" 和 "<JAVA_HOME>\include\win32";

Note 不同环境下的编译选项是不同的;

Windows下的gcc必须加上一个参数 "-Wl,--add-stdcall-alias -shared"; 
VC++的cl和Linux下的gcc不需要这个参数;

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gcc-Wl,--add-stdcall-alias-I"<JAVA_HOME>\include"-I"<JAVA_HOME>\include\win32"-shared -o hello.dll HelloJNI.cpp

>"-Wl"会把选项"--add-stdcall-alias"传输给链接器, 防止"UnsatisifiedLinkError".(一般导出的名字有"@nn"的前缀, 这个选项会把导出的名字加上没有前缀的别名)
有些时候也会使用 "-Wl,--kill-at".

>"-I"指定JNI头文件路径, 路径有空格时加上双引号.

可以使用nm命令列出函数导出的外部符号: 

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nm hello.dll |grepsay

>windows: "nm -g file.dll"

 

Step 4) 运行JNI程序

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java -Djava.library.path=. HelloJNI

>"-Djava.library.path=<path_to_lib>" 是可选的, 作为虚拟机的选项来制定动态库的路径.

Linux下可能需要设置路径:

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exportLD_LIBRARY_PATH=.

设置library路径为当前目录, 或者将so放入java执行目录下;

 

Other

编译链接相关

alias name that without @

"gcc -Wl,--add-stdcall-alias -I"C:\Program Files (x86)\Java\jdk1.7.0_17\include" -I"C:\Program Files (x86)\Java\jdk1.7.0_17\include\win32" -shared -o HelloWorld.dll HelloWorld.c"
"cl -I%java_home%\include -I%java_home%\include\win32 -LD HelloWorldImp.c -Fehello.dll"

Compile time and Link Time: -L, -I, -Wl,rpath=<your_lib_dir>

Linux: LD_LIBRARY_PATH; ldd; ldconfig; nm; readlf; Id; 

<refer to> http://blog.csdn.net/unbutun/article/details/6362474 & http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

 

JNI header

classpath: should point to the root folder where your top level package (JNI) goes to, not to the folder where your class is physically located.

http://stackoverflow.com/questions/14795050/javah-command-for-native-methods-gives-exception

1) "javah HelloWorld" (all the config is set)

2) "javah -o "JNIDemo.h" -jni -classpath "R:\Projects\JAVA\JavaJNIDemo\build\classes" javajnidemo.JavaJNIDemo"

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javah -o"<HeaderPATH>\JNIHeader.h" -jni -classpath "JavaClassPath" JNISample

 

JNI在Package里的case

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package myjni;//...Java codes

>JNI的类会被放入"myjni"的package内, 文件保存为"myjni\HelloJNI.java"

编译: 加上package(路径)名

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javac myjni\HelloJNI.java

javah: 使用package名, 把头文件生成到include文件夹下.

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javah -d include myini.HelloJNI

头文件的native函数: Java_<fully-qualified-name>_methodName

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JNIEXPORTvoid JNICALL Java_myjni_HelloJNI_sayHello(JNIEnv *, jobject);

---End---

 

Part2

JNI数据转换成C数据

e.g. jstring - GetStringUTFChars(), NewStringUTF(), ReleaseStringUTFChars()

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JNIEXPORTvoid JNICALL Java_JNISample_sampleFunction(JNIEnv* env, jobject obj, jstring name) 

{ 

    constchar* pname = env->GetStringUTFChars(name, NULL); 

    env->ReleaseStringUTFChars(name, pname); 

}

 

e.g. Array

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JNIEXPORT jint JNICALL Java_IntArray_sumArray  

        (JNIEnv *env, jobject obj, jintArray arr) {  

    jint buf[10];  

    jint i, sum = 0;  

    // This line is necessary, since Java arrays are not guaranteed  

    // to have a continuous memory layout like C arrays.  

    env->GetIntArrayRegion(arr, 0, 10, buf);  

    for(i = 0; i < 10; i++) {  

        sum += buf[i];  

    }  

    returnsum;  

}

<Refer to> http://ironurbane.iteye.com/blog/425513

JNI的数据定义

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// In "win\jni_mh.h" - machine header which is machine dependent

typedeflong            jint;

typedef__int64         jlong;

typedefsigned char    jbyte;

 

// In "jni.h"

typedefunsigned char  jboolean;

typedefunsigned short jchar;

typedefshort           jshort;

typedeffloat           jfloat;

typedefdouble          jdouble;

typedefjint            jsize;

 

 

C++ 调用Java方法

Read: http://stackoverflow.com/questions/819536/how-to-call-java-function-from-c

Windows http://public0821.iteye.com/blog/423941

Linux http://blog.sina.com.cn/s/blog_48eef8410100fjxr.html

 

JNI数据类型

 

Java TypeNative TypeDescriptionbooleanjboolean8 bits, unsignedbytejbyte8 bits, signedcharjchar16 bits, unsigneddoublejdouble64 bitsfloatjfloat32 bitsintjint32 bits, signedlongjlong64 bits, signedshortjshort16 bits, signedvoidvoidN/A

JNI的类型签名

Java TypeSignaturebooleanZbyteBcharCdoubleDfloatFintIlongJvoidVobjectLfully-qualified-class;type[][typemethod signature( arg-types) ret-type

e.g.

Java side

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classJNISample

{

    publicnative void launchSample();

    static

    {

        System.loadLibrary("Sample");

    }

 

    publicstatic int add(int a,intb) {

        returna+b;

    }

    publicboolean judge(booleanbool) {

        return!bool;

    }

}

C++side

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JNIEnv *env = GetJNIEnv();//Get env from JNI

jclass cls;

cls = env->FindClass("JNISample");

if(cls !=0)

{

    printf("find java class success\n");

    // constructor

    mid = env->GetMethodID(cls,"<init>","()V");

    if(mid !=0)

    {

        jobj=env->NewObject(cls,mid);

    }

 

    // static function

    mid = env->GetStaticMethodID( cls,"add", "(II)I");

    if(mid !=0)

    {

        square = env->CallStaticIntMethod( cls, mid, 5,5);

    }

 

    // function returns boolean

    mid = env->GetMethodID( cls,"judge","(Z)Z");

    if(mid !=0){

        jnot = env->CallBooleanMethod(jobj, mid, 1);

    }

}

 

查看属性和方法的签名

Java版本 "java -version"

反编译工具 javap: 

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javap -s -p -classpath R:\test.Demo

Check JNI version

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#ifdef JNI_VERSION_1_4    

printf("Version is 1.4 \n");  

#endif

使用API

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jint GetVersion(JNIEnv *env);

返回值需要转换, Need convert the result from DEC to HEX;

 

 

JNI实现过程中的Issue

 

x86 or x64 "Can't load load IA 32-bit dll on a amd 64 bit platform" 

确定本机上的默认JVM的版本和动态库的版本一致(x86或x64), Make sure JAVA's default path; check with "java -version" in command line.

3rdParty can't find dependent libraries 保证所依赖的动态库都能被找到;

1) copy the dll into executable file's folder 2) System.load() the dlls by dependecy orders

 

JNI_CreateJavaVM failed 

C++创建JVM调用Java方法 

http://docs.oracle.com/javase/1.4.2/docs/guide/jni/jni-12.html#JNI_CreateJavaVM & http://blog.csdn.net/louka/article/details/7318656

[我机器上装了多个版本的Java, 测试的时候没有成功]

jvm.dll(C:\Program Files (x86)\Java\jdk1.7.0_17\jre\bin\client; C:\Program Files (x86)\Java\jdk1.7.0_17\jre\bin\server; need check); jvm.lib(C:\Program Files (x86)\Java\jdk1.7.0_17\lib)

 

<Refer to> http://home.pacifier.com/~mmead/jni/cs510ajp/ & http://www.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html

Sample http://chnic.iteye.com/category/20179

 

JNI doc http://docs.oracle.com/javase/7/docs/technotes/guides/jni/ 

>JNA https://github.com/twall/jna/  XstartOnFirstThread

---End---

 

 

Part 3

启动Qt程序

通过Java启动Qt程序可以调用命令行, 这样Qt会在另一个进程开始.

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publicstatic void launchSampleApp() { 

   Runtime rn = Runtime.getRuntime(); 

   Process p =null; 

   try{ 

       String command ="QtAppSample"; 

       p = rn.exec(command); 

   }catch (Exception e) { 

       System.out.println("JAVA Failed to launch Sample."); 

   } 

}

>用进程启动Qt可能在通信效率和资源共享方面有些影响.

 

Qt事件循环是个dead loop, 如果直接在JNI中启动Qt程序会把Java的主线程Block住;  Qt main event loop will block the Java main thread;

Java 启动Qt需要另起一个线程

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classMain

{

    publicstatic JNISample sample =new JNISample();

    publicstatic void main(String[] args)

    {

        Thread t =new Thread(newRunnable() {

 

            publicvoid run() { 

                sample.launchSample(); 

            }      

        });    

        t.start();

    }

}

>JNISample的launchSample()函数是一个native方法

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publicnative void launchSample();

 

C++方面, 可以使用static instance的方式来引用Qt类;

Qt class: 类似singleton, 可以在JNI的cpp函数实现中引用静态的Qt的类来启动Qt程序;

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classQML_EXPORT QMLSample : publicQObject

{

    Q_OBJECT

 

public:

    staticQMLSample * GetInstance();

 

private:

    QMLSample ();

 

private:

    QDeclarativeView* mpView;

    JNIEnv* mpEnv;

    staticQMLSample * mpSInstance;

};

JNI函数启动Qt程序

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JNIEXPORTvoid JNICALL Java_JNISample_launchSample

  (JNIEnv *env, jobject obj)

{

    Q_UNUSED(obj);

 

    intargc = 0; char** argv = NULL;

    QApplication app(argc, argv);

    QMLSample::GetInstance()->Show();

    QMLSample::GetInstance()->SetJNIEnv(env);

    app.exec();

}

 

跨线程通信

signal/slot 

Java在子线程启动了Qt, 如果Java要向Qt发送消息的话, 需要使用signal/slot的方式.

Note 如果直接使用JNI调用Qt的directly方法, e.g. setWindowTitle(), Qt会报错: "setProperty : Cannot send events to objects owned by a different thread"

除了 1)signal/slot, 还可以显式使用 2)QMetaObject::invoke(), 利用MetaObject机制调用Qt函数

Note 信号发送方式需要改为 Qt::QueuedConnection (或者使用默认的AutoConnection)

e.g,2)

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constQMetaObject* metaObj = QMLSample::GetInstance()->metaObject();

intmethodIndex = metaObj->indexOfMethod("FunctionName(int,QString)");

QMetaMethod method = metaObj->method(methodIndex);

boolret = method.invoke(QMLDLLSample::GetInstance(),

                      Qt::AutoConnection,

                      Q_ARG(int, i),

                      Q_ARG(QString, string));

>这样就能跨线程调用Qt动态库的函数;

Note invoke的格式必须严格遵守, 多一个空格就错, must stictly follow the format, e.g.:metaObj->indexOfMethod("Function(int,QString)"), no space is allowed between "int," and "QString".

对于MetaObject无法识别的类型: 使用qRegisterMetaType()来注册: "QMetaMethod::invoke: Unable to handle unregistered datatype 'MyType'"

使用invoke异步调用函数的时候, 是无法得到return的返回值的: "It is unable to QMetaObject::invokeMethod with return values in queued connections"

Solution: 1) 把函数的参数改为指针, 来传递想要得到的值; ---由于是在异步的消息机制下, 这个也是不行的;
所以只能这样: 2) 得到值以后再发个消息....或者调用Java对象的方法传递值;

转载自：http://www.cnblogs.com/roymuste/p/3139583.html