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：
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-safepublic 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 finalprivate String name;public MutablePerson(String name, int age) { //rule 2: a factory method pattern is adopted to create the objectthis.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 blocke.printStackTrace();}}}}
运行结果：
joanieM1: 10year(s) oldjoanieM2: 20year(s) oldjoanieM2: 11year(s) oldjoanieM2: 11year(s) oldjoanieM2: 22year(s) oldjoanieM2: 22year(s) oldjoanieM2: 13year(s) oldjoanieM2: 13year(s) oldjoanieM2: 14year(s) oldjoanieM2: 14year(s) oldjoanieM1: 15year(s) oldjoanieM2: 25year(s) oldjoanieM1: 16year(s) oldjoanieM1: 16year(s) oldjoanieM2: 27year(s) oldjoanieM1: 27year(s) oldjoanieM1: 18year(s) oldjoanieM2: 28year(s) oldjoanieM2: 29year(s) oldjoanieM1: 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 onefinal class ImmutablePerson {private final int age; //Rule 1: all fields are private and finalprivate final String name;private ImmutablePerson(String name, int age) { //rule 2: a factory method pattern is adopted to create the objectthis.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 insteadpublic static ImmutablePerson getPerson(String name, int age) {return new ImmutablePerson(name, age);} }class TestThread extends Thread {final int MAX=10; final int idx;public TestThread(int idx) {this.idx = idx;}public void run() {for (int i = 0; i < MAX; i++) {ImmutableDemo.test = ImmutablePerson.getPerson("joanie"+idx, idx*MAX+i);try {Thread.sleep(20+i*2);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}}运行结果：joanie1: 10year(s) oldjoanie2: 20year(s) oldjoanie1: 11year(s) oldjoanie2: 21year(s) oldjoanie1: 12year(s) oldjoanie2: 22year(s) oldjoanie1: 13year(s) oldjoanie2: 23year(s) oldjoanie1: 14year(s) oldjoanie2: 24year(s) oldjoanie1: 15year(s) oldjoanie2: 25year(s) oldjoanie1: 16year(s) oldjoanie2: 26year(s) oldjoanie1: 17year(s) oldjoanie2: 27year(s) oldjoanie1: 18year(s) oldjoanie2: 28year(s) oldjoanie1: 19year(s) oldjoanie2: 29year(s) old
使用Immutable不得不提到的一个问题是：由于其使用创建新对象来代替setter/update，势必会造成过多垃圾回收的对象。因此，为了性能的考虑，往往在为某个对象提供其Immutable实现的同时，还需提供一个它的Mutable伴侣，就像StringBuffer相对于String，使用这些值对象时就需要综合考虑了。
参考资料：
1. Effective Java 第二版
2.
3. Java concurrency in Practice
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