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
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 + ‘\‘‘ +
‘}‘;
}
}
}
编译成class文件
javac com/wbo112/jni/JNIDemo.java
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。
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
执行java程序
java -Djava.library.path=./ com.wbo112.jni.JNIDemo
这个完全就是Oracle官方的例子了
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;
}
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);
}
}
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
执行生成的ctj文件
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$JAVA_HOME/jre/lib/amd64/server/
./ctj
原文：https://www.cnblogs.com/wbo112/p/15171914.html