java JNI介绍

java JNI介绍

目录

• java JNI介绍
  • 1、 Java调用C++代码
  • 2、C++代码调用java代码

JNI是Java Native Interface的全称。

oracle文档中是这样描述的

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.

翻译过来就是 JNI 是本机编程接口。 它允许在 Java 虚拟机 (VM) 内运行的 Java 代码与使用其他编程语言（例如 C、C++ 和汇编）编写的应用程序和库进行互操作。

说白了就是java代码和其他编程语言的代码相互调用。

今天就主要记录下日常的一些使用方式

1、 Java调用C++代码

1. 首先创建我们的java类 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);
       //保存JNI返回的结果
       private BlockingQueue<Entry> queue = new ArrayBlockingQueue<Entry>(10);
       //处理jni返回的任务 线程池
       private ExecutorService executor = Executors.newSingleThreadExecutor();
       public static void main(String[] args) throws InterruptedException {
           //加载so文件 libJNIDemo.so
           System.loadLibrary("JNIDemo");
           //System.load();   也可以用这种方式，参数是so文件的全路径
           JNIDemo jniDemo = new JNIDemo();
           //保存需要提交的任务
           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);
               //在这里会调用jni 提交任务，jni中会新启动一个线程，异步去执行任务，执行完了会调用putEntry方法，添加到需要回调的任务列表中
               jniDemo.sendStr(str);
           }
           Thread thread = new Thread(() -> {
               while (true) {
                   try {
                       Entry entry = jniDemo.queue.take();
                       sets.remove(entry.str);
                       //在线程中中，进行回调通知，比如向调用方发送任务处理结果
                       jniDemo.executor.execute(() -> System.out.println(entry.str + "process finish"));
                       //所有任务进行回调后，结束添加回调任务线程，同时关闭线程池
                       if (sets.isEmpty()) {
                           jniDemo.executor.shutdown();
                           break;
                       }
                   } catch (InterruptedException e) {
                       System.out.println("thread interrupt ");
                       jniDemo.executor.shutdown();
                       break;
                   }
               }
           });
           thread.start();
           //也可以通过这种方式，比如在其他地方在中断回调任务的添加执行，关闭线程池
           //thread.interrupt();
       }
       public boolean sendStr(String str) {
           return sendMsg(str);
       }
       public void putEntry(Entry entry) {
           queue.add(entry);
       }
       private static class Entry {
           //表示一个任务
           private String str;
           //表示任务处理结果
           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 + '\'' +
                       '}';
           }
       }
   }
   

2. 编译成class文件

   javac com/wbo112/jni/JNIDemo.java
   

3. 生成头文件

   javah com.wbo112.jni.JNIDemo
   

   这时就会在当前目录下生成com_wbo112_jni_JNIDemo.h。

4. 编写对应的cpp文件 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加载so文件的时候，就会调用到这个JNI_OnLoad
   //oracle文档这里有介绍：https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNJI_OnLoad
   jint JNI_OnLoad(JavaVM *jvm, void *reserved){
   	//这个表示java虚拟机,这个需要保存下来，因为JNIEnv只是当前线程有效，如果要在其他线程获取JNIEnv,就需要通过这个jvm来获取，后面有相应代码
   	vm=jvm;
   	//这个是为了模拟后面的回调时间，业务中应该不关注
   	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文件卸载的时候，会执行这个函数，在这里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
   	//这里有介绍，如果不是当前线程，是获取不到env的，这时，返回值就是JNI_EDETACHED,这时就需要调用AttachCurrentThread，给当前线程绑定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;
   	}
   	//这个是将jstring转成char*
   	const char *str = env->GetStringUTFChars(jstr, 0);
       std::cout << "start process task " + (std::string)str << std::endl;
   	//通过对象获取对应的类
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetObjectClass
   	jclass jcls=env->GetObjectClass(jobj);
   	//另一种方式获取对应的类
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#FindClass
   	jclass jentrycls=env->FindClass("com/wbo112/jni/JNIDemo$Entry");
   	//获取无参的构造方法
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID
   	jmethodID  jinit=env->GetMethodID(jcls,"<init>", "()V");
   	//调用构造方法获取对象
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewObject
   	jobject jentryobj=env->NewObject(jentrycls,jinit);
   	//获取entry类的str字段
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetFieldID
   	jfieldID jfieldStr=env->GetFieldID(jentrycls,"str","Ljava/lang/String;");
   	//给str字段设置值
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#Set_type_Field_routines
   	env->SetObjectField(jentryobj,jfieldStr,jstr);
   	char* msg = "Hello World!";
       jstring result = env->NewStringUTF(msg);
   	jfieldID jfieldResult=env->GetFieldID(jentrycls,"result","Ljava/lang/String;");
   	env->SetObjectField(jentryobj,jfieldResult,result);
   	//获取方法
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#GetMethodID
   	jmethodID jmtd=env->GetMethodID(jcls,"putEntry", "(Lcom/wbo112/jni/JNIDemo$Entry;)V");
   	//调用方法
   	//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;
   	//释放前面构造的char*
   	env->ReleaseStringUTFChars( jstr, str);
   	//释放全局对象
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#DeleteGlobalRef
   	env->DeleteGlobalRef(jobj);
   	env->DeleteGlobalRef(jstr);
   	//当前线程和jvm进行分离
   	//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对象是不能跨线程传递的，需要先转成全局引用
   	//https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#NewGlobalRef
   	jobject globalJobj=env->NewGlobalRef(jobj);
   	jobject globalJstr=env->NewGlobalRef(jstr);
   	//这里使用的是std::thread ,所以编译的时候需要加参数-std=c++11
      	std::thread t1(threadfunc,globalJobj,(jstring)globalJstr);
       //t1.join();
       t1.detach();
   	return 1;
   }
   

   编译cpp文件

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

5. 执行java程序

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

2、C++代码调用java代码

这个完全就是Oracle官方的例子了

The Invocation API (oracle.com)

1. 首先创建个ctj.cpp文件

    #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;
   	}
   

2. 再创建c++需要调用的java文件 Main.java

   public class Main {
       public static void test(int a) {
           System.out.println(" Main test:" + a);
       }
   }
   

3. 编译ctj.cpp文件

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

4. 执行生成的ctj文件

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