Java 线程第三版 第六章 高级同步议题 读书笔记
多线程数据同步错误比较难检测，因为通常是与事件的特定发生顺序有关。
一、同步术语
Barrier（屏障）
barrier是多个Thread的集合点：所有的Thread都应该到齐在这个barrier之后才能允许它们继续下去。
Condition variable(条件变量)
实际上不是变量，而是与某个lock有关联的变量。
Event variable（事件变量）
条件变量的另一个名称。
Critical section（临界区）
临界区是synchronized方法或者block。
Lock（锁）
已经进入synchronized方法或者block的特定Thread所授予的访问权。
Mutex（互斥）
lock的另一个名字。
Monitor(监视器)
在某些系统中，一个monitor就是一个lock，其他系统中的monitor类似于等待与通知的机制。
Reader/writer lock（读/写锁）
一种可以被多个Thread同时取得的锁，只要各Thread间都同意只从共享数据中做读的操作（或者写操作）。
Semaphore(信号量)
在各系统间使用是不一致的。不能完全等同于lock方式使用。
二、J2SE 5．0中加入的同步Class
Semaphore
Semaphore记录它可以发出的许可数目。
Semaphore与Lock的区别，Semaphore构造器需要制定许可数目，方法可以返回许可的总数与可用数量。Semaphore没有关联的条件变量可用。
Barrier
未广泛使用的原因是可以有两种替代方案：
1. 要求Thread等待条件变量，最后达到Thread通过通知所有其他Thread来释放掉barrier。
2. 通过join方法等待Thread终结，一旦所有的Thread已经被结合，就可以启动新的Thread进行程序的下一个阶段。
倒数的Latch
类似与Barrier的同步功能。区别在于释放的条件不是等待所有thread满足条件。
Exchanger
其结合barrier以及数据传递。能够让一对thread相互回合类似与barrier，这一对thread相遇时能够在分开前交换一组数据。
读/写锁
有时候需要在可能会很长时间的操作下从对象中读取信息。
三、防止死锁
避免死锁最简单的方法就是当持有一个锁的时候，绝对不再调用任何需要其他锁的方法。
死锁与自动释放的Lock
Lock Interface锁不会自动释放，即使异常退出程序。而synchronized关键字异常情况下依然会释放。
一种避免这种情况发生的办法就是把unLock放置到finally子句中。
synchronized在异常情况下都能释放锁并不一定在所有场景下都是优点，有可能当前方法执行一部分异常退出，其他线程再执行有可能获取到的并不是正确数据。
使用时限来防止死锁
通过在lock上设定期限，使用tryLock()在算法中提供替代方案。
四、死锁检测
死锁的问题在于它会导致程序无止境地等待。
支持检测死锁的Lock类，用于演示分析死锁过程：
此实现继承Lock Interface，所以可以用在任何需要Lock对象的地方。
一、同步术语
Barrier（屏障）
barrier是多个Thread的集合点：所有的Thread都应该到齐在这个barrier之后才能允许它们继续下去。
Condition variable(条件变量)
实际上不是变量，而是与某个lock有关联的变量。
Event variable（事件变量）
条件变量的另一个名称。
Critical section（临界区）
临界区是synchronized方法或者block。
Lock（锁）
已经进入synchronized方法或者block的特定Thread所授予的访问权。
Mutex（互斥）
lock的另一个名字。
Monitor(监视器)
在某些系统中，一个monitor就是一个lock，其他系统中的monitor类似于等待与通知的机制。
Reader/writer lock（读/写锁）
一种可以被多个Thread同时取得的锁，只要各Thread间都同意只从共享数据中做读的操作（或者写操作）。
Semaphore(信号量)
在各系统间使用是不一致的。不能完全等同于lock方式使用。
二、J2SE 5．0中加入的同步Class
Semaphore
Semaphore记录它可以发出的许可数目。
Semaphore与Lock的区别，Semaphore构造器需要制定许可数目，方法可以返回许可的总数与可用数量。Semaphore没有关联的条件变量可用。
Barrier
未广泛使用的原因是可以有两种替代方案：
1. 要求Thread等待条件变量，最后达到Thread通过通知所有其他Thread来释放掉barrier。
2. 通过join方法等待Thread终结，一旦所有的Thread已经被结合，就可以启动新的Thread进行程序的下一个阶段。
倒数的Latch
类似与Barrier的同步功能。区别在于释放的条件不是等待所有thread满足条件。
Exchanger
其结合barrier以及数据传递。能够让一对thread相互回合类似与barrier，这一对thread相遇时能够在分开前交换一组数据。
读/写锁
有时候需要在可能会很长时间的操作下从对象中读取信息。
三、防止死锁
import javax.swing.*;
import java.awt.event.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import javathreads.examples.ch03.*;
public class ScoreLabel extends JLabel implements CharacterListener {
private volatile int score = 0;
private int char2type = -1;
private CharacterSource generator = null, typist = null;
private Lock adminLock = new ReentrantLock();
private Lock charLock = new ReentrantLock();
private Lock scoreLock = new ReentrantLock();
public ScoreLabel (CharacterSource generator, CharacterSource typist) {
this.generator = generator;
this.typist = typist;
if (generator != null)
generator.addCharacterListener(this);
if (typist != null)
typist.addCharacterListener(this);
}
public ScoreLabel () {
this(null, null);
}
public void resetGenerator(CharacterSource newGenerator) {
try {
adminLock.lock();
if (generator != null)
generator.removeCharacterListener(this);
generator = newGenerator;
if (generator != null)
generator.addCharacterListener(this);
} finally {
adminLock.unlock();
}
}
public void resetTypist(CharacterSource newTypist) {
try {
adminLock.lock();
if (typist != null)
typist.removeCharacterListener(this);
typist = newTypist;
if (typist != null)
typist.addCharacterListener(this);
} finally {
adminLock.unlock();
}
}
public void resetScore() {
try {
charLock.lock();
scoreLock.lock();
score = 0;
char2type = -1;
setScore();
} finally {
charLock.unlock();
scoreLock.unlock();
}
}
private void setScore() {
// This method will be explained later in chapter 7
SwingUtilities.invokeLater(new Runnable() {
public void run() {
setText(Integer.toString(score));
}
});
}
public void newCharacter(CharacterEvent ce) {
try {
scoreLock.lock();
charLock.lock();
// Previous character not typed correctly - 1 point penalty
if (ce.source == generator) {
if (char2type != -1) {
score--;
setScore();
}
char2type = ce.character;
}
// If character is extraneous - 1 point penalty
// If character does not match - 1 point penalty
else {
if (char2type != ce.character) {
score--;
} else {
score++;
char2type = -1;
}
setScore();
}
} finally {
scoreLock.unlock();
charLock.unlock();
}
}
}
避免死锁最简单的方法就是当持有一个锁的时候，绝对不再调用任何需要其他锁的方法。
import javax.swing.*;
import java.awt.event.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import javathreads.examples.ch03.*;
public class ScoreLabel extends JLabel implements CharacterListener {
private volatile int score = 0;
private int char2type = -1;
private CharacterSource generator = null, typist = null;
private Lock adminLock = new ReentrantLock();
private Lock charLock = new ReentrantLock();
private Lock scoreLock = new ReentrantLock();
public ScoreLabel (CharacterSource generator, CharacterSource typist) {
this.generator = generator;
this.typist = typist;
if (generator != null)
generator.addCharacterListener(this);
if (typist != null)
typist.addCharacterListener(this);
}
public ScoreLabel () {
this(null, null);
}
public void resetGenerator(CharacterSource newGenerator) {
try {
adminLock.lock();
if (generator != null)
generator.removeCharacterListener(this);
generator = newGenerator;
if (generator != null)
generator.addCharacterListener(this);
} finally {
adminLock.unlock();
}
}
public void resetTypist(CharacterSource newTypist) {
try {
adminLock.lock();
if (typist != null)
typist.removeCharacterListener(this);
typist = newTypist;
if (typist != null)
typist.addCharacterListener(this);
} finally {
adminLock.unlock();
}
}
public void resetScore() {
try {
scoreLock.lock();
charLock.lock();
score = 0;
char2type = -1;
setScore();
} finally {
charLock.unlock();
scoreLock.unlock();
}
}
private void setScore() {
// This method will be explained later in chapter 7
SwingUtilities.invokeLater(new Runnable() {
public void run() {
setText(Integer.toString(score));
}
});
}
public void newCharacter(CharacterEvent ce) {
try {
scoreLock.lock();
charLock.lock();
// Previous character not typed correctly - 1 point penalty
if (ce.source == generator) {
if (char2type != -1) {
score--;
setScore();
}
char2type = ce.character;
}
// If character is extraneous - 1 point penalty
// If character does not match - 1 point penalty
else {
if (char2type != ce.character) {
score--;
} else {
score++;
char2type = -1;
}
setScore();
}
} finally {
scoreLock.unlock();
charLock.unlock();
}
}
}
死锁与自动释放的Lock
Lock Interface锁不会自动释放，即使异常退出程序。而synchronized关键字异常情况下依然会释放。
一种避免这种情况发生的办法就是把unLock放置到finally子句中。
synchronized在异常情况下都能释放锁并不一定在所有场景下都是优点，有可能当前方法执行一部分异常退出，其他线程再执行有可能获取到的并不是正确数据。
使用时限来防止死锁
通过在lock上设定期限，使用tryLock()在算法中提供替代方案。
四、死锁检测
死锁的问题在于它会导致程序无止境地等待。
支持检测死锁的Lock类，用于演示分析死锁过程：
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
//
// This is a very very slow implementation of a ReentrantLock class and is not for
// everyday usage. The purpose of this class is to test for deadlocks. The lock()
// method now throws a DeadlockDetectedException, if a deadlock occurs.
//
public class DeadlockDetectingLock extends ReentrantLock {
// List of deadlock detecting locks.
// This array is not thread safe, and must be externally synchronized
// by the class lock. Hence, it should only be called by static
// methods.
private static List deadlockLocksRegistry = new ArrayList();
private static synchronized void registerLock(DeadlockDetectingLock ddl) {
if (!deadlockLocksRegistry.contains(ddl))
deadlockLocksRegistry.add(ddl);
}
private static synchronized void unregisterLock(DeadlockDetectingLock ddl) {
if (deadlockLocksRegistry.contains(ddl))
deadlockLocksRegistry.remove(ddl);
}
// List of threads hard waiting for this lock.
// This array is not thread safe, and must be externally synchronized
// by the class lock. Hence, it should only be called by static
// methods.
private List hardwaitingThreads = new ArrayList();
private static synchronized void markAsHardwait(List l, Thread t) {
if (!l.contains(t)) l.add(t);
}
private static synchronized void freeIfHardwait(List l, Thread t) {
if (l.contains(t)) l.remove(t);
}
//
// Deadlock checking methods
//
// Given a thread, return all locks that are already owned
// Must own class lock prior to calling this method
private static Iterator getAllLocksOwned(Thread t) {
DeadlockDetectingLock current;
ArrayList results = new ArrayList();
Iterator itr = deadlockLocksRegistry.iterator();
while (itr.hasNext()) {
current = (DeadlockDetectingLock) itr.next();
if (current.getOwner() == t) results.add(current);
}
return results.iterator();
}
// Given a lock, return all threads that are hard waiting for the lock
// Must own class lock prior to calling this method
private static Iterator getAllThreadsHardwaiting(DeadlockDetectingLock l) {
return l.hardwaitingThreads.iterator();
}
// Check to see if a thread can perform a hard wait on a lock
private static synchronized
boolean canThreadWaitOnLock(Thread t, DeadlockDetectingLock l) {
Iterator locksOwned = getAllLocksOwned(t);
while (locksOwned.hasNext()) {
DeadlockDetectingLock current = (DeadlockDetectingLock) locksOwned.next();
// Thread can't wait if lock is already owned. This is the end condition
// for the recursive algorithm -- as the initial condition should be
// already tested for.
if (current == l) return false;
Iterator waitingThreads = getAllThreadsHardwaiting(current);
while (waitingThreads.hasNext()) {
Thread otherthread = (Thread) waitingThreads.next();
// In order for the thread to safely wait on the lock, it can't
// own any locks that have waiting threads that already owns
// lock. etc. etc. etc. recursively etc.
if (!canThreadWaitOnLock(otherthread, l)) {
return false;
}
}
}
return true;
}
//
// Core Constructors
//
public DeadlockDetectingLock() {
this(false, false);
}
public DeadlockDetectingLock(boolean fair) {
this(fair, false);
}
private boolean debugging;
public DeadlockDetectingLock(boolean fair, boolean debug) {
super(fair);
debugging = debug;
registerLock(this);
}
//
// Core Methods
//
public void lock() {
// Note: Owner can't change if current thread is owner. It is
// not guaranteed otherwise. Other owners can change due to
// condition variables.
if (isHeldByCurrentThread()) {
if (debugging) System.out.println("Already Own Lock");
super.lock();
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
return;
}
// Note: The wait list must be marked before it is tested because
// there is a race condition between lock() method calls.
markAsHardwait(hardwaitingThreads, Thread.currentThread());
if (canThreadWaitOnLock(Thread.currentThread(), this)) {
if (debugging) System.out.println("Waiting For Lock");
super.lock();
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
if (debugging) System.out.println("Got New Lock");
} else {
throw new DeadlockDetectedException("DEADLOCK");
}
}
//
// Note: It is debatable whether this is a hard or soft wait. Even if
// interruption is common, we don't know if the interrupting thread
// is also involved in the deadlock. As a compromise, we'll just
// not allow interrupts. This method is disabled.
public void lockInterruptibly() throws InterruptedException {
lock();
}
//
// Note: It is not necessary to override the tryLock() methods. These
// methods perform a soft wait -- there is a limit to the wait. It
// not possible to deadlock when locks are not waiting indefinitely.
//
// Note 1: Deadlocks are possible with any hard wait -- this includes
// the reacquitition of the lock upon return from an await() method.
// As such, condition variables will mark for the future hard
// wait, prior to releasing the lock.
// Note 2: There is no need to check for deadlock on this end because
// a deadlock can be created whether the condition variable owns the
// lock or is reacquiring it. Since we are marking *before* giving
// up ownership, the deadlock will be detected on the lock() side
// first. It is not possible to create a new deadlock just by releasing
// locks.
public class DeadlockDetectingCondition implements Condition {
Condition embedded;
protected DeadlockDetectingCondition(ReentrantLock lock, Condition embedded) {
this.embedded = embedded;
}
// Note: The algorithm can detect a deadlock condition if the thead is
// either waiting for or already owns the lock, or both. This is why
// we have to mark for waiting *before* giving up the lock.
public void await() throws InterruptedException {
try {
markAsHardwait(hardwaitingThreads, Thread.currentThread());
embedded.await();
} finally {
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
}
}
public void awaitUninterruptibly() {
markAsHardwait(hardwaitingThreads, Thread.currentThread());
embedded.awaitUninterruptibly();
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
}
public long awaitNanos(long nanosTimeout) throws InterruptedException {
try {
markAsHardwait(hardwaitingThreads, Thread.currentThread());
return embedded.awaitNanos(nanosTimeout);
} finally {
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
}
}
public boolean await(long time, TimeUnit unit) throws InterruptedException {
try {
markAsHardwait(hardwaitingThreads, Thread.currentThread());
return embedded.await(time, unit);
} finally {
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
}
}
public boolean awaitUntil(Date deadline) throws InterruptedException {
try {
markAsHardwait(hardwaitingThreads, Thread.currentThread());
return embedded.awaitUntil(deadline);
} finally {
freeIfHardwait(hardwaitingThreads, Thread.currentThread());
}
}
public void signal() {
embedded.signal();
}
public void signalAll() {
embedded.signalAll();
}
}
// Return a condition variable that support detection of deadlocks
public Condition newCondition() {
return new DeadlockDetectingCondition(this, super.newCondition());
}
//
// Testing routines here
//
// These are very simple tests -- more tests will have to be written
private static Lock a = new DeadlockDetectingLock(false, true);
private static Lock b = new DeadlockDetectingLock(false, true);
private static Lock c = new DeadlockDetectingLock(false, true);
private static Condition wa = a.newCondition();
private static Condition wb = b.newCondition();
private static Condition wc = c.newCondition();
private static void delaySeconds(int seconds) {
try {
Thread.sleep(seconds * 1000);
} catch (InterruptedException ex) {
}
}
private static void awaitSeconds(Condition c, int seconds) {
try {
c.await(seconds, TimeUnit.SECONDS);
} catch (InterruptedException ex) {
}
}
private static void testOne() {
new Thread(new Runnable() {
public void run() {
System.out.println("thread one grab a");
a.lock();
delaySeconds(2);
System.out.println("thread one grab b");
b.lock();
delaySeconds(2);
a.unlock(); b.unlock();
}
}).start();
new Thread(new Runnable() {
public void run() {
System.out.println("thread two grab b");
b.lock();
delaySeconds(2);
System.out.println("thread two grab a");
a.lock();
delaySeconds(2);
a.unlock(); b.unlock();
}
}).start();
}
private static void testTwo() {
new Thread(new Runnable() {
public void run() {
System.out.println("thread one grab a");
a.lock();
delaySeconds(2);
System.out.println("thread one grab b");
b.lock();
delaySeconds(10);
a.unlock(); b.unlock();
}
}).start();
new Thread(new Runnable() {
public void run() {
System.out.println("thread two grab b");
b.lock();
delaySeconds(2);
System.out.println("thread two grab c");
c.lock();
delaySeconds(10);
b.unlock(); c.unlock();
}
}).start();
new Thread(new Runnable() {
public void run() {
System.out.println("thread three grab c");
c.lock();
delaySeconds(4);
System.out.println("thread three grab a");
a.lock();
delaySeconds(10);
c.unlock(); a.unlock();
}
}).start();
}
private static void testThree() {
new Thread(new Runnable() {
public void run() {
System.out.println("thread one grab b");
b.lock();
System.out.println("thread one grab a");
a.lock();
delaySeconds(2);
System.out.println("thread one waits on b");
awaitSeconds(wb, 10);
a.unlock(); b.unlock();
}
}).start();
new Thread(new Runnable() {
public void run() {
delaySeconds(1);
System.out.println("thread two grab b");
b.lock();
System.out.println("thread two grab a");
a.lock();
delaySeconds(10);
b.unlock(); c.unlock();
}
}).start();
}
public static void main(String args[]) {
int test = 1;
if (args.length > 0)
test = Integer.parseInt(args[0]);
switch(test) {
case 1:
testOne(); // 2 threads deadlocking on grabbing 2 locks
break;
case 2:
testTwo(); // 3 threads deadlocking on grabbing 2 out of 3 locks
break;
case 3:
testThree(); // 2 threads deadlocking on 2 locks with CV wait
break;
default:
System.err.println("usage: java DeadlockDetectingLock [ test# ]");
}
delaySeconds(60);
System.out.println("--- End Program ---");
System.exit(0);
}
}
此实现继承Lock Interface，所以可以用在任何需要Lock对象的地方。
在所有需要检测死锁的地方都已此类取代，甚至包括synchronized关键字锁提供的锁。
在检测到死锁时会立即抛出DeadlockDetectedException。 但是这个类未考虑性能问题，所以尽量不要在真实环境中使用。
两个集合
lock通过使用registerLock、unregisterLock方法来进行添加与删除注册监听。
getAllLocksOwned、getAllThreadsHardwaiting用来取得之前所提过的两种等待子树。
canThreadWaitOnLock用来遍历等待树，检查某特定lock是否已经出现在树上。检测潜在死锁的主要方法。
覆写lock方法
如果当前thread已经持有此lock，没有理由检查死锁。仅需要调用super.lock()即可。
通过canThreadWaitOnLock判断是否死锁。如果死锁抛出异常，反之此Thread就会被添加到lock的hard wait列表中，并调用super.lock()方法。
五、Lock饥饿
每当有多个Thread争夺稀少的资源时，就会有饥饿的危机Thread会存在永远拿不到资源的状况。
每当有多个Thread争夺稀少的资源时，就会有饥饿的危机Thread会存在永远拿不到资源的状况。
例如：某个Thread获得锁之后永远不释放。
public ReentrantLock(boolean fair)
创建一个具有给定公平策略的 ReentrantLock。
这是一种非常接近先到先服务原则来授出Lock，不管Thread有多少或者是怎么写的，都不可能有任何Thread会因为Lock而挨饿。