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经典线程同步问题
生产者消费者问题
读者作家问题
哲学家吃饭问题
生产者消费者问题
http://en.wikipedia.org/wiki/Producer%E2%80%93consumer_problem
分别用锁、信号量、同步监视器模拟的例子。
package thread;import java.util.Random;import java.util.concurrent.Semaphore;import java.util.concurrent.locks.Condition;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;class Item { int id; public Item(int id) { this.id = id; } public String toString() { return "[item " + id + "]"; }}abstract class Buffer { int capacity; Item[] items; int count; int in, out; public Buffer(int capacity) { this.capacity = capacity; items = new Item[capacity]; count = 0; in = out = 0; } abstract void put(Item item); abstract Item get(); public void printBuf() { System.out.print("current buf status: [ "); for (int i = 0; i < capacity; i++) { System.out.print(items[i] + " "); } System.out.print("] "); System.out.print("count:" + count + " "); System.out.print("in:" + in + " out:" + out + " "); System.out.println(); }}/** * 利用锁实现线程同步的buffer * * @author jd * */class LockBuffer extends Buffer { Lock lock = new ReentrantLock(); Condition empty = lock.newCondition(); Condition full = lock.newCondition(); public LockBuffer(int capacity) { super(capacity); } public void put(Item item) { lock.lock(); try { while (count == capacity) full.await(); items[in] = item; in = (in + 1) % capacity; count++; empty.signal(); System.out.println(Thread.currentThread().getName() + " put item " + item.id); printBuf(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public Item get() { lock.lock(); Item res = null; try { while (count == 0) empty.await(); res = items[out]; items[out] = null; out = (out + 1) % capacity; count--; full.signal(); System.out.println(Thread.currentThread().getName() + " get item " + res.id); printBuf(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } return res; }}/** * 利用信号量实现的线程同步的buffer * * @author jd * */class SemaphoreBuffer extends Buffer { Semaphore mutex; Semaphore full; Semaphore empty; public SemaphoreBuffer(int capacity) { super(capacity); mutex = new Semaphore(1); full = new Semaphore(0); empty = new Semaphore(capacity); } public void put(Item item) { try { empty.acquire(); mutex.acquire(); } catch (InterruptedException e) { e.printStackTrace(); } items[in] = item; in = (in + 1) % capacity; count++; System.out.println(Thread.currentThread().getName() + " put item " + item.id); printBuf(); mutex.release(); full.release(); } public Item get() { try { full.acquire(); mutex.acquire(); } catch (InterruptedException e) { e.printStackTrace(); } Item res = items[out]; items[out] = null; out = (out + 1) % capacity; count--; System.out.println(Thread.currentThread().getName() + " get item " + res.id); printBuf(); mutex.release(); empty.release(); return res; }}/** * 利用同步监视器实现的线程同步的buffer * * @author jd * */class MonitorBuffer extends Buffer { public MonitorBuffer(int capacity) { super(capacity); } public void put(Item item) { synchronized (this) { try { while (count == capacity) wait(); } catch (InterruptedException e) { e.printStackTrace(); } items[in] = item; in = (in + 1) % capacity; count++; notifyAll(); System.out.println(Thread.currentThread().getName() + " put item " + item.id); printBuf(); } } public Item get() { synchronized (this) { try { while (count == 0) wait(); } catch (InterruptedException e) { e.printStackTrace(); } Item res = items[out]; items[out] = null; out = (out + 1) % capacity; count--; notifyAll(); System.out.println(Thread.currentThread().getName() + " get item " + res.id); printBuf(); return res; } }}class Producer implements Runnable { Buffer buf; Random rand = new Random(); public Producer(Buffer buf) { this.buf = buf; } public void run() { for (int i = 0; i < 10; i++) { Item item = new Item(rand.nextInt(100)); buf.put(item); try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } } }}class Consumer implements Runnable { Buffer buf; public Consumer(Buffer buf) { this.buf = buf; } public void run() { for (int i = 0; i < 10; i++) { Item item = buf.get(); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } }}public class BoundedBufferTest { public static void main(String[] args) { // Buffer buf = new LockBuffer(5); // Buffer buf = new SemaphoreBuffer(5); Buffer buf = new MonitorBuffer(5); // 3个生产者，3个消费者，每个生产或者消费10次。 for (int i = 0; i < 3; i++) { new Thread(new Producer(buf), "p" + i).start(); new Thread(new Consumer(buf), "c" + i).start(); } }}
读者作家问题
http://en.wikipedia.org/wiki/Readers%E2%80%93writers_problem
模拟第一读者问题。
package thread;import java.util.concurrent.Semaphore;/** * first reader writer problem, writer may starve; * * @author jd * */class Article { String content = "The original content."; Semaphore mutex, wrt; int readCount; public Article() { mutex = new Semaphore(1); wrt = new Semaphore(1); readCount = 0; } public String read() throws InterruptedException { mutex.acquire(); readCount++; if (readCount == 1) wrt.acquire(); mutex.release(); // reading is performed String res = content; System.out.println(Thread.currentThread().getName() + " is reading, the article is [" + res + "]"); Thread.sleep(1000); mutex.acquire(); readCount--; if (readCount == 0) wrt.release(); mutex.release(); return res; } public void Write(String str) throws InterruptedException { wrt.acquire(); // writing is performed content = "new content(" + str + ")"; System.out.println("content is changed to [" + content + "]"); Thread.sleep(2000); wrt.release(); }}class Reader implements Runnable { Article art; public Reader(Article a) { art = a; } public void run() { try { art.read(); } catch (InterruptedException e) { e.printStackTrace(); } }}class Writer implements Runnable { Article art; public Writer(Article a) { art = a; } public void run() { try { art.Write(Thread.currentThread().getName()); } catch (InterruptedException e) { e.printStackTrace(); } }}public class ReaderWriterTest { public static void main(String[] args) { Article art = new Article(); for (int i = 0; i < 3; i++) { new Thread(new Reader(art), "r" + i).start(); } // writer starves; new Thread(new Writer(art), "w").start(); for (int i = 0; i < 3; i++) { new Thread(new Reader(art), "rr" + i).start(); } }}
哲学家吃饭问题
http://en.wikipedia.org/wiki/Dining_philosophers_problem
模拟哲学家吃饭问题，该实现可能会产生死锁。
package thread;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** * This version may lead to deadlock. * * @author jd * */class Chopstick { private Lock lock = new ReentrantLock(); public void pickUp() { lock.lock(); } public void putDown() { lock.unlock(); }}class Philosopher extends Thread { private Chopstick left; private Chopstick right; public Philosopher(Chopstick left, Chopstick right) { this.left = left; this.right = right; } public void eat() { pickUp(); // eating System.out.println(Thread.currentThread().getName() + " is eating"); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } putDown(); System.out.println(Thread.currentThread().getName() + " done eating"); } public void pickUp() { System.out.println(Thread.currentThread().getName() + " is trying to pick up left chopstick.."); left.pickUp(); // this sleep makes all philosopers pick up left chopstick and try to // pick up the right, which leads to a deadlock. // this rarely happens, but it can do. try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " is trying to pick up right chopstick.."); right.pickUp(); } public void putDown() { left.putDown(); right.putDown(); } public void run() { eat(); }}public class DiningPhilosopherTest { public static void main(String[] args) { Chopstick[] chopsticks = new Chopstick[5]; for (int i = 0; i < 5; i++) chopsticks[i] = new Chopstick(); for (int i = 0; i < 5; i++) { new Philosopher(chopsticks[i], chopsticks[(i + 1) % 5]).start(); } }}
不死锁版本，当某个哲学家无法拿到两只筷子时，会主动放弃已有资源（筷子）而放弃本次吃饭。
package thread;import java.util.Random;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantLock;/** * * @author jd * */class ChopstickNoDeadLock { private Lock lock = new ReentrantLock(); public boolean pickUp() { return lock.tryLock(); } public void putDown() { lock.unlock(); }}class PhilosopherNoDeadLock extends Thread { Random rand = new Random(); private ChopstickNoDeadLock left; private ChopstickNoDeadLock right; public PhilosopherNoDeadLock(ChopstickNoDeadLock left, ChopstickNoDeadLock right) { this.left = left; this.right = right; } public void eat() { if (pickUp()) { pickUp(); // eating System.out.println(Thread.currentThread().getName() + " is eating"); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } putDown(); System.out.println(Thread.currentThread().getName() + " done eating"); } else { System.out.println(Thread.currentThread().getName() + " gives up eating"); } } public boolean pickUp() { System.out.println(Thread.currentThread().getName() + " is trying to pick up left chopstick.."); if (!left.pickUp()) { return false; } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " is trying to pick up right chopstick.."); if (!right.pickUp()) { left.putDown(); return false; } return true; } public void putDown() { left.putDown(); right.putDown(); } public void run() { // for (int i = 0; i < 10; i++) { eat(); // try { // Thread.sleep(rand.nextInt(10) * 100); // } catch (InterruptedException e) { // e.printStackTrace(); // } // } }}public class DiningPhilosopherTestNoDeadLock { public static void main(String[] args) { ChopstickNoDeadLock[] chopsticks = new ChopstickNoDeadLock[5]; for (int i = 0; i < 5; i++) chopsticks[i] = new ChopstickNoDeadLock(); for (int i = 0; i < 5; i++) { new PhilosopherNoDeadLock(chopsticks[i], chopsticks[(i + 1) % 5]).start(); } }}
经典线程同步问题
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