Java的语言特点是什么(java语言的主要特点是什么)
623
2022-05-30
AQS
(1)aqs
使用node实现fifo队列,可以用于构建锁或者其他的同步装置的基础框架
利用了一个int类型表示状态
使用方法是继承
子类通过继承并通过实现它的方法管理其状态{acquire 和 release}
可以同时实现排他锁和共享锁模式(独占,共享)
(2)CountDownLatch
/**
* 一般用于 当主线程需要等待 子线程执行完成之后 再执行的场景
* * 线程名称 子线程结束
* * thread1 ---------------------- end |
* * thread2 ---------------------- end | 主线程 主线程结束
* * thread3 ---------------------- end | --------------------- end
* * thread4 ---------------------- end |
* * thread5 ---------------------- end |
* *
*
*/
@Slf4j
public class CountDownLatchExample1 {
private static int threadCount = 200;
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newCachedThreadPool();
final CountDownLatch countDownLatch = new CountDownLatch(threadCount);
for (int i = 0; i < threadCount; i++) {
final int threadNum = i;
service.execute(() -> {
try {
test(threadNum);
} catch (InterruptedException e) {
log.error("exception",e);
} finally {
countDownLatch.countDown();
}
});
}
// countDownLatch.await();
countDownLatch.await(10, TimeUnit.MILLISECONDS); //超时等待
log.info("finish");
service.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(100);
}
}
(3)Semaphore
/**
* 一般用于控制同一时刻运行的线程数量
* | |
* ---------|----- |---------
* | |
* ---------|----- |---------
* | |
*/
@Slf4j
public class SemaphoreExample1 {
private static int threadCount = 20;
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(3);
for (int i = 0; i < threadCount; i++) {
final int threadNum = i;
service.execute(() -> {
try {
//semaphore.acquire(); 每次拿一个许可证
//semaphore.acquire(3); 每次拿三个许可证
semaphore.tryAcquire(3);
semaphore.tryAcquire(1,TimeUnit.SECONDS); //尝试一秒之内获取许可
semaphore.tryAcquire(3,1,TimeUnit.SECONDS);
if(semaphore.tryAcquire()) { //尝试获取许可证 ,没有获取到直接将当前线程丢弃
test(threadNum);
semaphore.release();
}else {
log.info(Thread.currentThread().getName()+"我没有拿到许可证,┭┮﹏┭┮");
}
} catch (InterruptedException e) {
log.error("exception", e);
}
});
}
service.shutdown();
}
private static void test(int threadNum) throws InterruptedException {
log.info("{}", threadNum);
Thread.sleep(1000);
}
}
(4)CyclicBarrier
/**
* 一般用于多个线程之间相互等待,当全部都到达某个屏障点的时候在,继续执行每个线程,并且可以重复循环使用
* 线程名称 某个屏障点 终点
* thread1 ------------| ---------- end
* thread2 ------------| ---------- end
* thread3 ------------| ---------- end
* thread4 ------------| ---------- end
* thread5 ------------| ---------- end
*
*/
@Slf4j
public class CyclicBarrierExample1 {
// private static CyclicBarrier cyclicBarrier = new CyclicBarrier(5);
private static CyclicBarrier cyclicBarrier = new CyclicBarrier(5,() -> {
log.info("五个线程都已经准备就绪");
});
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newCachedThreadPool();
for (int i = 0; i < 10; i++) {
final int threadNum = i;
Thread.sleep(1000);
service.execute(() -> {
try {
race(threadNum);
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
});
}
service.shutdown();
}
private static void race(int threadNum) throws InterruptedException, BrokenBarrierException {
Thread.sleep(1000);
log.info("{} is ready", threadNum);
cyclicBarrier.await();
try {
// cyclicBarrier.await(1, TimeUnit.SECONDS);
} catch (Exception e) {
// e.printStackTrace();
}
log.info("{} continue", threadNum);
}
}
(5)锁
锁的简单使用:
/**
* class Point {
* * private double x, y;
* * private final StampedLock sl = new StampedLock();
* *
* * void move(double deltaX, double deltaY) { // an exclusively locked method
* * long stamp = sl.writeLock();
* * try {
* * x += deltaX;
* * y += deltaY;
* * } finally {
* * sl.unlockWrite(stamp);
* * }
* * }
* *
* * double distanceFromOrigin() { // A read-only method
* * long stamp = sl.tryOptimisticRead();
* * double currentX = x, currentY = y;
* * if (!sl.validate(stamp)) {
* * stamp = sl.readLock();
* * try {
* * currentX = x;
* * currentY = y;
* * } finally {
* * sl.unlockRead(stamp);
* * }
* * }
* * return Math.sqrt(currentX * currentX + currentY * currentY);
* * }
* *
* * void moveIfAtOrigin(double newX, double newY) { // upgrade
* * // Could instead start with optimistic, not read mode
* * long stamp = sl.readLock();
* * try {
* * while (x == 0.0 && y == 0.0) {
* * long ws = sl.tryConvertToWriteLock(stamp);
* * if (ws != 0L) {
* * stamp = ws;
* * x = newX;
* * y = newY;
* * break;
* * }
* * else {
* * sl.unlockRead(stamp);
* * stamp = sl.writeLock();
* * }
* * }
* * } finally {
* * sl.unlock(stamp);
* * }
* * }
* * }}
* *
*/
@Slf4j
public class LockExample1 {
private static int clientTotal = 5000;
private static int threadTotal = 200;
private static int count = 0;
//重入锁
private final static Lock lock = new ReentrantLock();
//重入读写锁
private final static Map
private final static ReentrantReadWriteLock reenLock = new ReentrantReadWriteLock();
private final static Lock readLock = reenLock.readLock();
private final static Lock writeLock = reenLock.writeLock();
//stamped锁
private final static StampedLock stampedLock = new StampedLock();
//condition
private final static ReentrantLock REENTRANT_LOCK = new ReentrantLock();
private final static Condition condition = REENTRANT_LOCK.newCondition();
//重入读写锁的使用
public Integer getValue(Integer key) {
readLock.lock();
Integer value = null;
try {
value = map.get(key);
} catch (Exception e) {
e.printStackTrace();
} finally {
readLock.unlock();
}
return value;
}
//重入读写锁的使用
public Set
Set
readLock.lock();
try {
set = map.keySet();
} catch (Exception e) {
e.printStackTrace();
} finally {
readLock.unlock();
}
return set;
}
/**
* 重入读写锁的使用
* @param key
* @param value
* @return
*/
public Integer put(Integer key, Integer value) {
writeLock.lock();
try {
map.put(key, value);
} catch (Exception e) {
e.printStackTrace();
} finally {
writeLock.unlock();
}
return value;
}
/**
* condition的使用
*/
public static void testCond(){
new Thread(() -> {
try {
REENTRANT_LOCK.lock();
log.info("运动员获取锁");
condition.await();
log.info("运动员接收到信号,比赛开始~~~~");
}catch (Exception e){
e.printStackTrace();
}finally {
REENTRANT_LOCK.unlock();
}
}).start();
new Thread(() -> {
try {
REENTRANT_LOCK.lock();
log.info("裁判获取锁");
Thread.sleep(3000);
log.info("裁判发送信号");
condition.signalAll();
}catch (Exception e){
e.printStackTrace();
}finally {
REENTRANT_LOCK.unlock();
}
}).start();
}
public static void main(String[] args) throws InterruptedException {
ExecutorService service = Executors.newCachedThreadPool();
final Semaphore semaphore = new Semaphore(threadTotal);
final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
for (int i = 0; i < clientTotal; i++) {
service.execute(() -> {
try {
semaphore.acquire();
add();
semaphore.release();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
countDownLatch.countDown();
}
});
}
countDownLatch.await();
service.shutdown();
log.info("count {}", count);
testCond();
}
/**
* stampedLock的使用方式
*/
private static void add() {
long stamp = stampedLock.writeLock();
try {
count++;
} catch (Exception e) {
} finally {
stampedLock.unlock(stamp);
}
}
}
同步锁synchronized不是JUC中的锁但也顺便提下,它是由synchronized 关键字进行同步,实现对竞争资源互斥访问的锁。
同步锁的原理:对于每一个对象,有且仅有一个同步锁;不同的线程能共同访问该同步锁。在同一个时间点该同步锁能且只能被一个线程获取到,其他线程都得等待。
另外:synchronized是Java中的关键字且是内置的语言实现;它是在JVM层面上实现的,不但可以通过一些监控工具监控synchronized的锁定,而且在代码执行时出现异常,JVM会自动释放锁定;synchronized等待的线程会一直等待下去,不能响应中断。
重入锁ReentrantLock,顾名思义:就是支持重进入的锁,它表示该锁能够支持一个线程对资源的重复加锁。另外该锁孩纸获取锁时的公平和非公平性选择,所以它包含公平锁与非公平锁(它们两也可以叫可重入锁)。首先提出两个疑问:它怎么实现重进入呢?释放逻辑还跟AQS中一样吗?
ReentrantLock 独有的功能
可指定是公平锁还是非公共锁,公平锁就是 先等待的线程先获得锁
提供了一个condition类,可以分组唤醒需要的线程,
提供了能够中断等待锁的线程机制,lock.lockInterruptibly()
非公平锁
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
// 同步状态已经被其他线程占用,则判断当前线程是否与被占用的线程是同一个线程,如果是同一个线程则允许获取,并state+1
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
该方法增加了再次获取同步状态的处理逻辑:通过判断当前线程是否为获取锁的线程来决定获取操作是否成功。如果是获取锁的线程再次请求,则将同步状态值进行增加并返回true,表示获取同步状态成功。
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
上面代码是释放锁的代码。如果该锁被获取了n次,那么前(n-1)次都是返回false,直至state=0,将占有线程设置为null,并返回true,表示释放成功。
公平锁
公平锁与非公平锁有啥区别呢? 还是从源码中分析吧。
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
// 区别:增加判断同步队列中当前节点是否有前驱节点的判断
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
// 一样支持重入
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
与非公平锁的唯一不同就是增加了一个判断条件:判断同步队列中当前节点是否有前驱节点的判断,如果方法返回true,则表示有线程比当前线程更早地请求获取锁,因此需要等待前驱线程获取并释放锁之后才能继续获取锁。
公平锁与非公平锁的区别
从上面源码中得知,公平性锁保证了锁的获取按照FIFO原则,但是代价就是进行大量的线程切换。而非公平性锁,可能会造成线程“饥饿”(不会保证先进来的就会先获取),但是极少线程的切换,保证了更大的吞吐量。下面我们看下案例:
import org.junit.Test;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
public class FairAndUnfairTest {
private static Lock fairLock = new ReentrantLock2(true);
private static Lock unFairLock = new ReentrantLock2(false);
@Test
public void fair() throws Exception{
testLock(fairLock);
}
@Test
public void unFairLock() throws Exception{
testLock(unFairLock);
}
private static void testLock(Lock lock) throws InterruptedException, ExecutionException {
ExecutorService threadPool = Executors.newFixedThreadPool(5);
List
for (int i = 0 ; i < 5; i++) {
Future
list.add(future);
}
long cost = 0;
for (Future
cost += future.get();
}
// 查看五个线程所需耗时的时间
System.out.println("cost:" + cost + " ms");
}
private static class Job implements Callable
private Lock lock;
public Job(Lock lock) {
this.lock = lock;
}
@Override
public Long call() throws Exception {
long st = System.currentTimeMillis();
// 同一线程获取100锁
for (int i =0; i < 100; i ++) {
lock.lock();
try {
System.out.println("Lock by[" + Thread.currentThread().getId() + "]," +
"Waiting by[" + printThread(((ReentrantLock2)lock).getQueuedThreads()) + "]");
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
// 返回100次所需的时间
return System.currentTimeMillis() - st;
}
private String printThread(Collection
StringBuilder ids = new StringBuilder();
for (Thread t : list) {
ids.append(t.getId()).append(",");
}
return ids.toString();
}
}
private static class ReentrantLock2 extends ReentrantLock {
public ReentrantLock2(boolean fair) {
super(fair);
}
public Collection
List
Collections.reverse(arrayList);
return arrayList;
}
}
}
非公平性锁的测试结果,cost:117 ms
公平性锁的测试结果,cost:193 ms
读写锁
读写锁维护了一对锁,一个读锁和一个写锁,通过分离读锁和写锁,使得并发性相比一般的排他锁(同一时刻只允许一个线程进行访问)有了很大的提升。
下面我们看下它有啥特性:
特性
说明
公平性选择
可重入
该锁支持可重进入。
读线程在获取了读锁之后能够再次获取读锁。
写线程在获取了写锁之后能够再次获取写锁。
锁降级
遵循获取写锁、获取读锁在释放写锁的次序,写锁能够降级成读锁。
排他性
当写线程访问时,其他读写线程均被阻塞
另外读写锁是采取一个整型变量来维护多种状态。高16位表示读,低16位表示写。
// 偏移位
static final int SHARED_SHIFT = 16;
static final int SHARED_UNIT = (1 << SHARED_SHIFT);
// 读写线程允许占用的最大数
static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
// 独占标志
static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
下面从源码中找出这些特性,先看下写锁的实现:
1 protected final boolean tryAcquire(int acquires) {
2
3 Thread current = Thread.currentThread();
4 int c = getState();
5 // 表示独占个数,也就是与低16位进行与运算。
6 int w = exclusiveCount(c);
7 if (c != 0) {
8 // c!=0 且 w==0表示不存在写线程,但存在读线程
9 if (w == 0 || current != getExclusiveOwnerThread())
10 return false;
11 if (w + exclusiveCount(acquires) > MAX_COUNT)
12 throw new Error("Maximum lock count exceeded");
13 /**
14 * 获取写锁的条件:
15 * 不能存在读线程且当前线程是当前占用锁的线程(这里体现可重入性和排他性);
16 * 当前占用锁的次数不能超过最大数
17 */
18 setState(c + acquires);
19 return true;
20 }
21 if (writerShouldBlock() ||
22 !compareAndSetState(c, c + acquires))
23 return false;
24 setExclusiveOwnerThread(current);
25 return true;
26 }
27 static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
获取读锁源码如下:
protected final int tryAcquireShared(int unused) {
Thread current = Thread.currentThread();
int c = getState();
/**
* exclusiveCount(c) != 0: 表示有写线程在占用
* getExclusiveOwnerThread() != current : 当前占用锁的线程不是当前线程。
* 如果上面两个条件同时满足,则获取失败。
* 上面表明如果当前线程是拥有写锁的线程可以获取读锁(体现可重入和锁降级)。
*/
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c + SHARED_UNIT)) {
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
return 1;
}
return fullTryAcquireShared(current);
}
Java 任务调度
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