synchronized方法和synchronized代码段
synchronized方法 只有一个线程能访问，只有这个方法执行完之后其他线程才能访问
synchronized方法的锁是当前对象，也就是如果new多次是不能阻止多个线程访问代码的
public class SyncTest {
/**
* 对于SyncTest的对象 只有一个线程能访问
* 对于不同的SyncTest的对象可以有多个线程访问
*/
public synchronized void say(){
System.out.println("hello get the lock!");
//休眠200s 不释放锁
try {
Thread.sleep(200 * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 与synchronized方法相同
*/
public void say(){
synchronized(this) {
System.out.println("hello get the lock!");
//休眠200s 不释放锁
try {
Thread.sleep(200 * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 同一时间只有一个线程能访问
* 与synchronized方法不同
**/
public void say(){
synchronized(SyncTest.class) {
System.out.println("hello get the lock!");
//休眠200s 不释放锁
try {
Thread.sleep(200 * 1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
最为线程安全类 最常见的做法就是加synchronized方法
StringBuilder
StringBuffer
HashTable
HashMap
Thread类常用方法
名称 new Thread(String name) new Thread(Runnable run,String name)
获取当前Thread Thread.currentThread();
获取当前所有Thread的数量 Thread.activeCount();
线程变量ThreadLocal
ThreadLocal是一个特殊的模板变量 每个线程获取到的都是自己的值
public class ThreadLocalTest{
private static ThreadLocal<ThreadLocalTest> threadLocal=new ThreadLocal<>();
private static ThreadLocalTest mainThreadLocal;
/**
* 创建当前线程的ThreadLocal对象
*/
public static void prepare(){
if(threadLocal.get()==null){
threadLocal.set(new ThreadLocalTest());
}
}
/**
* 主线程中调用
*/
public static void prepareMain(){
if(mainThreadLocal!=null){
throw new RuntimeException("只有一个线程可以调用prepareMain");
}
mainThreadLocal=new ThreadLocalTest();
if(threadLocal.get()==null){
threadLocal.set(mainThreadLocal);
}
}
public static ThreadLocalTest getMain(){
return mainThreadLocal;
}
public static ThreadLocalTest myThreadTest(){
return threadLocal.get();
}
private ThreadLocalTest(){
}
}
其实现原理 利用的是Thread.currentThread();
concurrent包的CountDownLatch
一个辅助类，初始化的时候指定数据个数，没调用一次countDown()数字减少一个
await方法会阻塞 直到CountDownLatch里的数字为0
CountDownLatch countDownLatch=new CountDownLatch(100);
for(int i=0;i<100;i++){
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(200);
System.out.println(Thread.currentThread().getName()+" done!");
countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("所有线程完成了!");
线程A , B ,C。 A,B运行完后运行C
CountDownLatch countDownLatch=new CountDownLatch(2);
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" done !");
countDownLatch.countDown();
}
},"A").start();
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" done !");
countDownLatch.countDown();
}
},"B").start();
new Thread(new Runnable() {
@Override
public void run() {
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+" done !");
}
},"C").start();
线程池初步
long startTime=System.currentTimeMillis();
CountDownLatch countDownLatch=new CountDownLatch(65535);
for(int i=0;i<65535;i++){
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(200);
countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("所有线程完成了!"+(System.currentTimeMillis()-startTime)+"ms");
上面的代码预计消耗时间是200ms但是 实际远远超出了我们的预期，这是因为新建线程消耗了一部分时间，cpu拥挤，并不会有那么多线程同时运行，会排队一会。
通过线程池可以优化上述代码的速度
/*
* 空闲线程
*/
public class IdleThread extends Thread {
public LinkedBlockingQueue<Runnable> queue;
public boolean started = true;
public IdleThread(LinkedBlockingQueue<Runnable> queue) {
this.queue = queue;
}
@Override
public void run() {
while (started) {
try {
Runnable runnable = queue.poll(100, TimeUnit.MILLISECONDS);
if(runnable!=null)
runnable.run();
} catch (InterruptedException e) {
}
}
}
public void kill() {
started = false;
}
}
/**
* 基础线程池
*/
public class ThreadPool {
/**
* 池最小大小
*/
private int minSize;
/**
* 池最大大小
*/
private int maxSize;
LinkedBlockingQueue<Runnable> tasks;
List<IdleThread> idleThreads;
public ThreadPool(int size){
this.maxSize=size;
this.minSize=size;
init();
}
private void init(){
tasks=new LinkedBlockingQueue<>(minSize);
idleThreads=new ArrayList<>(minSize);
for(int i=0;i<minSize;i++){
IdleThread thread=new IdleThread(tasks);
idleThreads.add(thread);
thread.start();
}
}
public void execute(Runnable runnable) throws InterruptedException {
tasks.put(runnable);
}
public void shutdown(){
try {
for (IdleThread idleThread : idleThreads) {
idleThread.kill();
idleThread.interrupt();
}
}catch (Exception e){
e.printStackTrace();;
}
tasks.clear();
}
public void waitForAll(){
while(tasks.size()>0){
try {
Thread.sleep(0,1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
通过上述两类 重写一下上面的代码
long startTime=System.currentTimeMillis();
CountDownLatch countDownLatch=new CountDownLatch(65535);
ThreadPool threadPool=new ThreadPool(9000);
for(int i=0;i<65535;i++){
threadPool.execute(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(200);
countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
try {
countDownLatch.await();
threadPool.shutdown();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("所有线程完成了!"+(System.currentTimeMillis()-startTime)+"ms");
可以看到速度比那个优化很多
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