//: concurrency/CriticalSection.java
// Synchronizing blocks instead of entire methods. Also
// demonstrates protection of a non-thread-safe class
// with a thread-safe one.
//package concurrency;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;
//: concurrency/CriticalSection.java
// Synchronizing blocks instead of entire methods. Also
// demonstrates protection of a non-thread-safe class
// with a thread-safe one.
//package concurrency;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;
class Pair { // Not thread-safe
private int x, y;
public Pair(int x, int y) {
this.x = x;
this.y = y;
}
public Pair() { this(0, 0); }
public int getX() { return x; }
public int getY() { return y; }
public void incrementX() { x++; }
public void incrementY() { y++; }
public String toString() {
return "x: " + x + ", y: " + y;
}
public class PairValuesNotEqualException
extends RuntimeException {
public PairValuesNotEqualException() {
super("Pair values not equal: " + Pair.this);
}
}
// Arbitrary invariant -- both variables must be equal:
public void checkState() {
if(x != y)
throw new PairValuesNotEqualException();
}
}
// Protect a Pair inside a thread-safe class:
abstract class PairManager {
AtomicInteger checkCounter = new AtomicInteger(0);
protected Pair p = new Pair();
private List<Pair> storage =
Collections.synchronizedList(new ArrayList<Pair>());
public synchronized Pair getPair() {
// Make a copy to keep the original safe:
return new Pair(p.getX(), p.getY());
}
// Assume this is a time consuming operation
protected void store(Pair p) {
storage.add(p);
try {
TimeUnit.MILLISECONDS.sleep(50);
} catch(InterruptedException ignore) {}
}
public abstract void increment();
}
// Synchronize the entire method:
class PairManager1 extends PairManager {
public synchronized void increment() {
p.incrementX();
p.incrementY();
store(getPair());
}
}
// Use a critical section:
class PairManager2 extends PairManager {
public void increment() {
Pair temp;
synchronized(this) {
p.incrementX();
p.incrementY();
temp = getPair();
}
store(temp);
}
}
class PairManipulator implements Runnable {
private PairManager pm;
public PairManipulator(PairManager pm) {
this.pm = pm;
}
public void run() {
while(true)
pm.increment();
}
public String toString() {
return "Pair: " + pm.getPair() +
" checkCounter = " + pm.checkCounter.get();
}
}
class PairChecker implements Runnable {
private PairManager pm;
public PairChecker(PairManager pm) {
this.pm = pm;
}
public void run() {
while(true) {
pm.checkCounter.incrementAndGet();
pm.getPair().checkState(); //此处可能没有互斥,getPair()使用的是synchronized提供的对象锁,其他锁不起作用
}
}
}
public class CriticalSection {
// Test the two different approaches:
static void
testApproaches(PairManager pman1, PairManager pman2) {
ExecutorService exec = Executors.newCachedThreadPool();
PairManipulator
pm1 = new PairManipulator(pman1),
pm2 = new PairManipulator(pman2);
PairChecker
pcheck1 = new PairChecker(pman1),
pcheck2 = new PairChecker(pman2);
exec.execute(pm1);
exec.execute(pm2);
exec.execute(pcheck1);
exec.execute(pcheck2);
try {
TimeUnit.MILLISECONDS.sleep(500);
} catch(InterruptedException e) {
System.out.println("Sleep interrupted");
}
System.out.println("pm1: " + pm1 + "\npm2: " + pm2);
System.exit(0);
}
public static void main(String[] args) {
PairManager
pman1 = new PairManager1(),
pman2 = new PairManager2();
testApproaches(pman1, pman2);
}
} /* Output: (Sample)
pm1: Pair: x: 15, y: 15 checkCounter = 272565
pm2: Pair: x: 16, y: 16 checkCounter = 3956974
*///:~
//: concurrency/ExplicitCriticalSection.java
// Using explicit Lock objects to create critical sections.
//package concurrency;
import java.util.concurrent.locks.*;
// Synchronize the entire method:
class ExplicitPairManager1 extends PairManager {
private Lock lock = new ReentrantLock();
public Pair getPair() { //覆盖基类方法，统一使用Lock锁
// Make a copy to keep the original safe:
lock.lock();
try {
return new Pair(p.getX(), p.getY());
} finally {
lock.unlock();
}
}
public synchronized void increment() {
lock.lock();
try {
p.incrementX();
p.incrementY();
store(getPair());
} finally {
lock.unlock();
}
}
}
// Use a critical section:
class ExplicitPairManager2 extends PairManager {
private Lock lock = new ReentrantLock();
public void increment() {
Pair temp;
lock.lock(); //与对象pm2的getPair()中持有锁的方式不同
try {
p.incrementX();
p.incrementY();
temp = getPair();
} finally {
lock.unlock();
}
store(temp);
}
}
public class ExplicitCriticalSection {
public static void main(String[] args) throws Exception {
PairManager
pman1 = new ExplicitPairManager1(),
pman2 = new ExplicitPairManager2();
CriticalSection.testApproaches(pman1, pman2);
}
} /* Output: (Sample)
pm1: Pair: x: 15, y: 15 checkCounter = 174035
pm2: Pair: x: 16, y: 16 checkCounter = 2608588
*///:~
明确下synchronized的几个关键点：
A．无论synchronized关键字加在方法上还是对象上，他取得的锁都是对象，而不是把一段代码或函数当作锁――而且同步方法很可能还会被其他线程的对象访问。
B．每个对象只有一个锁（lock）和之相关联。
C．实现同步是要很大的系统开销作为代价的，甚至可能造成死锁，所以尽量避免无谓的同步控制。
synchronized可以加在方法上，也可以加在对象上，通常理解为，只有持有了锁才可以进行对应代码块的执行。
java.util.concurrent.locks包下面提供了一些锁的实现，有读写锁，公平锁等。
将synchronized替换成lock的实现可以提升性能：
1. 大部分应用场景是读写互斥，写和写互斥，读和读不互斥。而synchronized则是都互斥。
可以利用读写锁来优化性能，读锁锁住读的代码块，写锁锁住写的代码块。
2. 要确保你在理解原来利用到synchronized的代码逻辑，避免一概而论地把synchronized替换成锁。
public void getPair(){
return "x="+x + ",y="+y;
}
//这个函数
public synchronized void increment() {
x++;
y++;
getPair();
}
//可以替换成
public void increment() {
lock.lock();
try{
x++;
y++;
getPair();
}finally{
lock.unlock();
}
//但是,如果getPair()是synchronized
public synchronized void getPair(){
return "x="+x + ",y="+y;
}
//还能替换这个函数吗？
public synchronized void increment() {
x++;
y++;
getPair();
}
//这时候就不能简单地使用lock来替换了，这里要调用getPair();必需申请到对象锁，这个时候increment也要竞争这把锁
//因此这里的代码效果是读写互斥。
//如果只是用lock来锁住increment，则达不到效果。还得同时锁getPair();
//这里嵌套了synchronized，而synchronized的嵌套结构中在同一个对象的方法上是共享一把锁的。
//上面只是简单的例子，java编程思想的多线程编程中有更详细的这个例子，有兴趣的可以看看。
}
