一、能避免同步吗？

1. 寄存器的效应

从逻辑上的观点来看，每一个Thread都有自己的一组寄存器。当操作系统将某个Thread分配给CPU时，它会把该Thread特有的信息载入到CPU的寄存器中。

在分配不同的Thread给CPU之前，它会将寄存器的信息存下来。

所以Thread间绝不会共享保存在寄存器的数据。

当虚拟机进入synchronized方法或者块时。它必须又一次载入本来已经缓存到自有寄存器上的数据。在虚拟机离开synchroized方法或者块之前，它必须把自有寄存器存入主寄存器中。

2. 重排语句的效应

多个线程间调度运行的无序性就是重排语句的效应。

3. 双重检查的Locking

二、Atomic变量

1. Atomic Class的概述

    一些不支持。比如字符或者浮点。
    AtomicStampedReference可以让mark或stamp跟在不论什么对象的引用上。
    AtomicMarkableReference提供一个包括对象引用结合boolean的数据结构。

2. 使用Atomic Class

import javax.swing.*;
import java.awt.event.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import javathreads.examples.ch05.*;

public class ScoreLabel extends JLabel implements CharacterListener {
    private AtomicInteger score = new AtomicInteger(0);
    private AtomicInteger char2type = new AtomicInteger(-1);
    private AtomicReference<CharacterSource> generator = null;
    private AtomicReference<CharacterSource> typist = null;

    public ScoreLabel (CharacterSource generator, CharacterSource typist) {
        this.generator = new AtomicReference(generator);
        this.typist = new AtomicReference(typist);

        if (generator != null)
             generator.addCharacterListener(this);
        if (typist != null)
             typist.addCharacterListener(this);       
    }

    public ScoreLabel () {
        this(null, null);
    }

    public void resetGenerator(CharacterSource newGenerator) {
        CharacterSource oldGenerator;

        if (newGenerator != null)
            newGenerator.addCharacterListener(this);

        oldGenerator = generator.getAndSet(newGenerator);
        if (oldGenerator != null)
            oldGenerator.removeCharacterListener(this);
    }

    public void resetTypist(CharacterSource newTypist) {
        CharacterSource oldTypist;

        if (newTypist != null)
            newTypist.addCharacterListener(this);

        oldTypist = typist.getAndSet(newTypist);
        if (oldTypist != null)
            oldTypist.removeCharacterListener(this);
    }

    public void resetScore() {
        score.set(0);
        char2type.set(-1);
        setScore();
    }

    private void setScore() {
        // This method will be explained later in chapter 7
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                setText(Integer.toString(score.get()));
            }
        });
    }

    public void newCharacter(CharacterEvent ce) {
        int oldChar2type;
 
        // Previous character not typed correctly - 1 point penalty
        if (ce.source == generator.get()) {
            oldChar2type = char2type.getAndSet(ce.character);

            if (oldChar2type != -1) {
                score.decrementAndGet();
                setScore();
            }
        }
        // If character is extraneous - 1 point penalty
        // If character does not match - 1 point penalty
        else if (ce.source == typist.get()) {
            while (true) {
                oldChar2type = char2type.get();

                if (oldChar2type != ce.character) {
                    score.decrementAndGet();
                    break;
                } else if (char2type.compareAndSet(oldChar2type, -1)) {
                    score.incrementAndGet();
                    break;
                }
            }

            setScore();
        }
    } 
}

通知与Atomic变量

import java.awt.*;
import javax.swing.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import javathreads.examples.ch05.*;

public class AnimatedCharacterDisplayCanvas extends CharacterDisplayCanvas implements CharacterListener, Runnable {

    private AtomicBoolean done = new AtomicBoolean(true);
    private AtomicInteger curX = new AtomicInteger(0);
    private AtomicInteger tempChar = new AtomicInteger(0);
    private Thread timer = null;

    public AnimatedCharacterDisplayCanvas() {
        startAnimationThread();
    }

    public AnimatedCharacterDisplayCanvas(CharacterSource cs) {
        super(cs);
        startAnimationThread();
    }

    private void startAnimationThread() {
        if (timer == null) {
            timer = new Thread(this);
            timer.start();
        }
    }

    public void newCharacter(CharacterEvent ce) {
        curX.set(0);
        tempChar.set(ce.character);
        repaint();
    }

    protected void paintComponent(Graphics gc) {
        char[] localTmpChar = new char[1];
        localTmpChar[0] = (char) tempChar.get();
        int localCurX = curX.get();

        Dimension d = getSize();
        int charWidth = fm.charWidth(localTmpChar[0]);
        gc.clearRect(0, 0, d.width, d.height);
        if (localTmpChar[0] == 0)