java JNI介紹

java JNI介紹
- 1、 Java呼叫C++程式碼
- 2、C++程式碼呼叫java程式碼

JNI是Java Native Interface的全稱。

oracle文件中是這樣描述的

The JNI is a native programming interface. It allows Java code that runs inside a Java Virtual Machine (VM) to interoperate with applications and libraries written in other programming languages, such as C, C++, and assembly.

翻譯過來就是 JNI 是本機程式設計介面。它允許在 Java 虛擬機器 (VM) 內執行的 Java 程式碼與使用其他程式語言（例如 C、C++ 和彙編）編寫的應用程式和庫進行互操作。

說白了就是java程式碼和其他程式語言的程式碼相互呼叫。

今天就主要記錄下日常的一些使用方式

1、 Java呼叫C++程式碼

首先建立我們的java類 JNIDemo.java

package com.wbo112.jni;

import java.util.*;

import java.util.concurrent.*;

public class JNIDemo {

    public native String callHello();

    public native boolean sendMsg(String str);

    //儲存JNI返回的結果

    private BlockingQueue<Entry> queue = new ArrayBlockingQueue<Entry>(10);

    //處理jni返回的任務 執行緒池

    private ExecutorService executor = Executors.newSingleThreadExecutor();

    public static void main(String[] args) throws InterruptedException {

        //載入so檔案 libJNIDemo.so

        System.loadLibrary("JNIDemo");

        //System.load();   也可以用這種方式，引數是so檔案的全路徑

        JNIDemo jniDemo = new JNIDemo();

        //儲存需要提交的任務

        Set<String> sets = new ConcurrentSkipListSet<>();

        String str;

        for (int i = 0; i < 10; i++) {

            str = UUID.randomUUID().toString();

            sets.add(str);

            System.out.println("commit task :" + str);

            //在這裡會呼叫jni 提交任務，jni中會新啟動一個執行緒，非同步去執行任務，執行完了會呼叫putEntry方法，新增到需要回調的任務列表中

            jniDemo.sendStr(str);

        }

        Thread thread = new Thread(() -> {

            while (true) {

                try {

                    Entry entry = jniDemo.queue.take();

                    sets.remove(entry.str);

                    //線上程中中，進行回撥通知，比如向呼叫方傳送任務處理結果

                    jniDemo.executor.execute(() -> System.out.println(entry.str + "process finish"));

                    //所有任務進行回撥後，結束添加回調任務執行緒，同時關閉執行緒池

                    if (sets.isEmpty()) {

                        jniDemo.executor.shutdown();

                        break;

                    }

                } catch (InterruptedException e) {

                    System.out.println("thread interrupt ");

                    jniDemo.executor.shutdown();

                    break;

                }

            }

        });

        thread.start();

        //也可以通過這種方式，比如在其他地方在中斷回撥任務的新增執行，關閉執行緒池

        //thread.interrupt();

    }

    public boolean sendStr(String str) {

        return sendMsg(str);

    }

    public void putEntry(Entry entry) {

        queue.add(entry);

    }

    private static class Entry {

        //表示一個任務

        private String str;

        //表示任務處理結果

        private String result;

        public String getStr() {

            return str;

        }

        public void setStr(String str) {

            this.str = str;

        }

        public String getResult() {

            return result;

        }

        public void setResult(String result) {

            this.result = result;

        }

        @Override

        public String toString() {

            return "Entry{" +

                    "str='" + str + '\'' +

                    ", result='" + result + '\'' +

                    '}';

        }

    }

}

編譯成class檔案
```
javac com/wbo112/jni/JNIDemo.java
```
生成標頭檔案
```
javah com.wbo112.jni.JNIDemo
```
這時就會在當前目錄下生成com_wbo112_jni_JNIDemo.h。

編寫對應的cpp檔案 JNIDemo1.cpp

#include "com_wbo112_jni_JNIDemo.h"

#include <iostream>

#include <thread>

#include <cstdlib>

#include <ctime>

#include <unistd.h>

JavaVM *vm=NULL;

unsigned seed;

//在java載入so檔案的時候，就會呼叫到這個JNI_OnLoad

//oracle文件這裡有介紹：https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNJI_OnLoad

jint JNI_OnLoad(JavaVM *jvm, void *reserved){

	//這個表示java虛擬機器,這個需要儲存下來，因為JNIEnv只是當前執行緒有效，如果要在其他執行緒獲取JNIEnv,就需要通過這個jvm來獲取，後面有相應程式碼

	vm=jvm;

	//這個是為了模擬後面的回撥時間，業務中應該不關注

	seed = time(0);

    srand(seed);

	return JNI_VERSION_1_8;

}

//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNI_OnUnload

//so檔案解除安裝的時候，會執行這個函式，在這裡面可以做一些清理工作

void JNI_OnUnload_L(JavaVM *vm, void *reserved){

	vm=NULL;

}

void threadfunc(jobject jobj,jstring jstr)

{

	JNIEnv*	env = NULL;

	sleep(rand() % 10);

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#GetEnv

	//這裡有介紹，如果不是當前執行緒，是獲取不到env的，這時，返回值就是JNI_EDETACHED,這時就需要呼叫AttachCurrentThread，給當前執行緒繫結env

	jint status = vm->GetEnv((void **)&env, JNI_VERSION_1_8);

    	if (status == JNI_EDETACHED) {

			//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#AttachCurrentThread

			if (vm->AttachCurrentThread((void **)&env, NULL)!= JNI_OK){

				return;

			}

		} else if(status!= JNI_OK){

		std::cout<<"getEnv err"<<std::endl;

		return;

	}

	//這個是將jstring轉成char*

	const char *str = env->GetStringUTFChars(jstr, 0);

    std::cout << "start process task " + (std::string)str << std::endl;

	//通過物件獲取對應的類

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetObjectClass

	jclass jcls=env->GetObjectClass(jobj);

	//另一種方式獲取對應的類

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#FindClass

	jclass jentrycls=env->FindClass("com/wbo112/jni/JNIDemo$Entry");

	//獲取無參的構造方法

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID

	jmethodID  jinit=env->GetMethodID(jcls,"<init>", "()V");

	//呼叫構造方法獲取物件

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewObject

	jobject jentryobj=env->NewObject(jentrycls,jinit);

	//獲取entry類的str欄位

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetFieldID

	jfieldID jfieldStr=env->GetFieldID(jentrycls,"str","Ljava/lang/String;");

	//給str欄位設定值

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#Set_type_Field_routines

	env->SetObjectField(jentryobj,jfieldStr,jstr);

	char* msg = "Hello World!";

    jstring result = env->NewStringUTF(msg);

	jfieldID jfieldResult=env->GetFieldID(jentrycls,"result","Ljava/lang/String;");

	env->SetObjectField(jentryobj,jfieldResult,result);

	//獲取方法

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID

	jmethodID jmtd=env->GetMethodID(jcls,"putEntry", "(Lcom/wbo112/jni/JNIDemo$Entry;)V");

	//呼叫方法

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#Call_type_Method_routines

	env->CallVoidMethod(jobj,jmtd,jentryobj);

	std::cout<<" end process task  " + (std::string)str<<std::endl;

	//釋放前面構造的char*

	env->ReleaseStringUTFChars( jstr, str);

	//釋放全域性物件

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#DeleteGlobalRef

	env->DeleteGlobalRef(jobj);

	env->DeleteGlobalRef(jstr);

	//當前執行緒和jvm進行分離

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#DetachCurrentThread

	vm->DetachCurrentThread();

}	

JNIEXPORT jstring JNICALL Java_com_wbo112_jni_JNIDemo_callHello

  (JNIEnv *env, jobject jobj){

	char* msg = "Hello World!";

	jstring result = env->NewStringUTF(msg); // C style string to Java String

 	return result;

}

JNIEXPORT jboolean JNICALL Java_com_wbo112_jni_JNIDemo_sendMsg

  (JNIEnv *env, jobject jobj, jstring jstr){

	//jobject物件是不能跨執行緒傳遞的，需要先轉成全域性引用

	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewGlobalRef

	jobject globalJobj=env->NewGlobalRef(jobj);

	jobject globalJstr=env->NewGlobalRef(jstr);

	//這裡使用的是std::thread ,所以編譯的時候需要加引數-std=c++11

   	std::thread t1(threadfunc,globalJobj,(jstring)globalJstr);

    //t1.join();

    t1.detach();

	return 1;

}

編譯cpp檔案

g++ -shared -std=c++11  -I $JAVA_HOME/include/linux -I $JAVA_HOME/include  -fPIC -o libJNIDemo.so JNIDemo1.cpp

執行java程式

java -Djava.library.path=./ com.wbo112.jni.JNIDemo

2、C++程式碼呼叫java程式碼

這個完全就是Oracle官方的例子了

The Invocation API (oracle.com)

首先建立個ctj.cpp檔案

 #include <jni.h>       /* where everything is defined */

    int main(){

    JavaVM *jvm;       /* denotes a Java VM */

    JNIEnv *env;       /* pointer to native method interface */

    JavaVMInitArgs vm_args; /* JDK/JRE 6 VM initialization arguments */

    JavaVMOption* options = new JavaVMOption[1];

    options[0].optionString = "-Djava.class.path=./";

    vm_args.version = JNI_VERSION_1_8;

    vm_args.nOptions = 1;

    vm_args.options = options;

    vm_args.ignoreUnrecognized = false;

    /* load and initialize a Java VM, return a JNI interface

 *      * pointer in env */

    JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

    delete options;

    /* invoke the Main.test method using the JNI */

    jclass cls = env->FindClass("Main");

    jmethodID mid = env->GetStaticMethodID(cls, "test", "(I)V");

    env->CallStaticVoidMethod(cls, mid, 100);

    /* We are done. */

    jvm->DestroyJavaVM();

    return 0;

	}

再建立c++需要呼叫的java檔案 Main.java

public class Main {

    public static void test(int a) {

        System.out.println(" Main test:" + a);

    }

}

編譯ctj.cpp檔案

g++ -I $JAVA_HOME/include/linux -I $JAVA_HOME/include   -L"$JAVA_HOME/jre/lib/amd64/server/" ctj.cpp -o  ctj   -ljvm

執行生成的ctj檔案

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$JAVA_HOME/jre/lib/amd64/server/

./ctj

java JNI介紹

java JNI介紹

java JNI介紹

1、 Java呼叫C++程式碼

2、C++程式碼呼叫java程式碼

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