使用Immutable对象解决线程安全

何为Immutable对象？

简单地说，如果一个对象实例不能被更改就是一个Immutable的对象，Java SDK提供的大量值对象，比如String等都是Immutable的对象。

如何使对象Immutable？

按照Effective Java的说明，需要满足下面几条规则：

保证类不能被继承- 为了避免其继承的类进行mutable的操作
移调所有setter/update等修改对象实例的操作
保证所有的field是private和final的
为什么要采用Immutable对象？

在并发程序中，使用Immutable可以既保证线程安全性，跟并发锁方式相比，它大大增强了并发时的效率。尤其当一个对象是值对象时，更应该考虑采用Immutable方式。

为了说明，这里先举一个Mutable的非线程安全的例子。person应该是一个典型的值对象，但下面的例子没有使他具备Immutable特性。

//non thread-safe

public class ImmutableDemo {
    static MutablePerson testM = new MutablePerson("joanieM", 14);

    public static void main(String[] args) {
        Thread t1 = new MutableTestThread(1);
        t1.start();
        Thread t2 = new MutableTestThread(2);
        t2.start();
    }
}

class MutablePerson {
    private int age; //Rule 1: all fields are private and final
    private String name;

    public MutablePerson(String name, int age) { //rule 2: a factory method pattern is adopted to create the object
        this.age = age;
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String toString() {
        return name +": "+age +"year(s) old";
    }

    public void updatePerson(String name, int age){
        this.name = name;
        this.age = age;
        System.out.println(this);
    }
}

class MutableTestThread extends Thread {
    final int MAX=10;
    final int idx;

    public MutableTestThread(int idx) {
        this.idx = idx;
    }

    public void run() {
        for (int i = 0; i < MAX; i++) {
            ImmutableDemo.testM.updatePerson("joanieM"+idx, idx*MAX+i);

            try {
                Thread.sleep(20+i*2);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}
运行结果：
joanieM1: 10year(s) old
joanieM2: 20year(s) old
joanieM2: 11year(s) old
joanieM2: 11year(s) old
joanieM2: 22year(s) old
joanieM2: 22year(s) old
joanieM2: 13year(s) old
joanieM2: 13year(s) old
joanieM2: 14year(s) old
joanieM2: 14year(s) old
joanieM1: 15year(s) old
joanieM2: 25year(s) old
joanieM1: 16year(s) old
joanieM1: 16year(s) old
joanieM2: 27year(s) old
joanieM1: 27year(s) old
joanieM1: 18year(s) old
joanieM2: 28year(s) old
joanieM2: 29year(s) old
joanieM1: 19year(s) old
结果中红颜色标注的都是错误的结果。由于没有采取任何保证线程安全性的操作，首先线程t1执行完updatePerson函数的this.age=age后被挂起，线程t2执行完updatePerson函数的this.name=name后被挂起，线程t1继续执行，此时的name值为t2执行的结果joanieM2，age则为t1执行的结果，于是打印出了错误的值：joanieM2: 11year(s) old

下面的例子给出了如何使用Immutable来保证值对象的线程安全性的。

public class ImmutableDemo {
    //test is a shared thread-safe object.
    static ImmutablePerson test = ImmutablePerson.getPerson("joanie", 14);

    public static void main(String[] args) {
        Thread t1 = new TestThread(1);
       t1.start();
        Thread t2 = new TestThread(2);
        t2.start();
    }
}

//a sample immutable class
//Rule 4: define class as final one
final class ImmutablePerson {
    private final int age; //Rule 1: all fields are private and final
    private final String name;

    private ImmutablePerson(String name, int age) { //rule 2: a factory method pattern is adopted to create the object
        this.age = age;
        this.name = name;
        System.out.println(this);
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String toString() {
        return name +": "+age +"year(s) old";
    }
    //Rule 3: no setters for value update. Create a new class instead
    public static ImmutablePerson getPerson(String name, int age) {
        return new ImmutablePerson(name, age);
    }
}

class TestThread extends Thread {
    final int MAX=10;

补充：综合编程 , 安全编程 ,