JAVA 多线程设计模式
^^^^^^^^^^^^^^^^^^^
- 作者：臭豆腐[trydofor.com]
- 日期：2008-12-24
- 授权：署名-非商业-保持一致 1.0 协议
- 声明：拷贝、分发、呈现和表演本作品，请保留以上全部信息。
0. 文档目录
^^^^^^^^^^
[[<=$INDEX]]
1. 适用范围与知识背景
^^^^^^^^^^^^^^^^^^^^^
本文是对"Java多线程设计模式",中国铁道出版社 (ISBN 7-113-06402-7)
一书的学习备忘,适用于java1.4.
实际项目中应该尽量回避多线程,如果必须使用,最好使用DougLee大师的杰作.
让我也具有DougLee一样能执行多线程的计算机头脑吧 :D
2. Thread 状态图
^^^^^^^^^^^^^^^^
================= txt figure-1 : Thread Status =================
(_) Thread Status /jdk1.4
| new www.trydofor.com 2008-12-21
+------------+ =============================
| initial | * Object's method/status
+-----+------+ @ Thread's method/status
| @start() synchronized can not timeout
+------------+
| executable |<------------------------------------+
+-+----------+ @sleep() +----------+ @timeout |
| ^ +-------->|@sleeping +------------>|
| @yield | | +----------+ @interrupt()|
v | | |
+----------+-+ |*wait() +----------+ |
| running |--+-------->|*wait-set | @timeout |
+-----+------+ | +----+-----+ |
| | | *notify/All() |
| @run() | | @interrupt() |
| | synch- v |
v | ronized +----------+ acquire |
+------------+ +-------->|*race-set |------------>|
| final | | +----------+ |
+-----+------+ | +----------+ done |
| sys gc +-------->| blocking +------------>|
(8) block-io +----------+
================================================================
3. 脆弱独木桥(SingleThreadExecution)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
脆弱的独木桥,又称危险区(Critical Section),只能承重一个人,
因此一次只能允许一个线程执行.
当共享资源可以被多个线程访问,且资源状态会改变时,就可以考虑该模式.
对危险区的划分是重点,以人脑模拟电脑思考程序的执行过程.
同时要注意死锁和继承等对安全性的破坏.
如果看到 synchronized 相互嵌套,基本上就会死锁了.
以下代码和Thread.sleep()很像 :0
================== java :ThreadSleep.java ======================
public class ThreadSleep {
public static void sleep(long ms) throws InterruptedException{
if(ms>0){
Object obj = new Object();
synchronized(obj){
obj.wait(ms);
}
}
}
}
================================================================
4. 只许参观不许摸(Immutable)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
像java.lang.String一样,实例一旦产生,状态就不再改变.
对于一个只读的东东,即便同时有无数个线程访问,也没什么问题.
final class/method/field/parameter 可以很大的提高安全性.
值得注意的是,不要出现以下代码中的问题,错把 mutable 数据暴露出去.
====================== java : Mutable.java =====================
public final class Mutable {
private final StringBuffer sb;
public Mutable(String str){
sb = new StringBuffer(str);
}
public StringBuffer getText(){
return sb;
}
}
================================================================
5. 不见不散(GuardedSuspension)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
如果条件不满足,就一直等,不见不散.
便于记忆,可以理解成多线程版的 if
====================== java : Queue.java =======================
import java.util.LinkedList;
public final class Queue {
private final LinkedList queue = new LinkedList();
public synchronized Object get(){ // 为什么同步this
while(queue.size()<=0){ // 为什么不用If
try{
wait(); // 为什么不是Thread.sleep()
}catch(InterruptedException e){
// can not be interrupted
}
}
return queue.removeFirst();
}
public synchronized void put(Object obj){
queue.addLast(obj);
notifyAll();
}
}
================================================================
6. 你们聊,我有事先走了(Balking)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
不见不散固然为人所敬仰,但不是所有时候都需要不见不散.比如,
* 不需要去执行时.
比如我经常神经性的Ctrl+s,但编辑器只有当文件变更时,才真正保持.
* 不想等到条件成立时.
小兔说:我妈妈叫我小兔兔.
小猪说:我妈妈叫我小猪猪.
小鸡说:你们聊,我有事先走了!
* 只能执行一次时.
发工资了,如果能多领几份该多好啊 :D
把Balking的状态通知给调用者的方式.
* 忽略balk的发生.
* 以返回值表达,比如true/false
* 以异常的形式通知.
======================= java : Data.java =======================
public abstract class Data {
private boolean changed;
public final synchronized void change(){
changed = true;
}
public final synchronized void save(){
if(!changed) return;
doSomething();
changed = false;
}
protected abstract void doSomething();
}
================================================================
7. 生产者消费者(ProducerConsumer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
生产者和消费者在不同的线程上运行,一般情况下,两者的处理速度存在很大差异.
因此在两者之间存在一个隐藏的"桥梁"来缓解这样的差异.管道(pipe)模式,是该
模式的简化形式.
所有参与者,
* Data 被传来传去的东西.
* Producer 生产Data的家伙.
* Customer 消灭Data的家伙.
* Channel 以上三者的纽带,是传送,调度的核心.
为什么Producer不能直接把Data给Customer呢?
因为这样的话,就是Producer线程来完成Customer的动作了.
参与者多了,有必要设置超时和可取消.好的习惯是让Chanel互斥的方法 throws
InterruptedException,隐式的告诉调用者,该方法具有以下含义,
* 我可能很耗时
* 我可以被取消
* 我可能调用了Object.wait(),Thread.sleep(),Thread.join().
注意,Object.wait()被interrupt的时候,需要重新获得锁,才会被执行.
====================== java : Channel.java =====================
import java.util.LinkedList;
public class Channel {
private final int capacity;
private final LinkedList datas;
public Channel(int capacity){
this.capacity = capacity;
this.datas = new LinkedList();
}
public synchronized void put(Object obj) throws InterruptedException{
while(datas.size()>=capacity){
wait();
}
datas.addLast(obj);
notifyAll();
}
public synchronized Object get() throws InterruptedException{
while(datas.size()<= 0){
wait();
}
Object obj = datas.removeFirst();
notifyAll();
return obj;
}
}
================================================================
8. 读写锁(ReadWriteLock)
^^^^^^^^^^^^^^^^^^^^^^^^
为了解决以下两种冲突,
* 读写(read-write conflict)
* 写写(write-write conflict)
在read和write的时候,都采用以下的结构,
===================== java : Example.java ======================
ReadWriteLock lock = new ReadWriteLock();
public String read() throws InterruptedException{
lock.readLock(); // 为什么不可以在try内呢?
try{
return doRead();
}finally{
lock.readUnlock();
}
}
public void write(String str) throws InterruptedException{
lock.writeLock();
try{
doWrite(str);
}finally{
lock.writeUnlock();
}
}
================================================================
================== java : ReadWriteLock.java ===================
public final class ReadWriteLock {
private int readingReaders = 0;
private int waitingWriters = 0; // 不计数会有什么问题?
private int writingWriters = 0;
private boolean preferWriter = true; // 为什么添加这个标记?
public synchronized void readLock() throws InterruptedException{
while(writingWriters>0 || (preferWriter && waitingWriters>0)){
wait();
}
readingReaders++;
}
public synchronized void readUnlock() throws InterruptedException{
readingReaders --;
preferWriter = true;
notifyAll(); // 可能唤醒哪些线程(Reader/Writer)?
}
public synchronized void writeLock() throws InterruptedException{
waitingWriters ++;
try{
while(readingReaders>0|| writingWriters>0){
wait();
}
}finally{
waitingWriters --;
}
writingWriters ++;
}
public synchronized void writeUnlock() throws InterruptedException{
writingWriters --;
preferWriter = false;
notifyAll(); // 可能唤醒哪些线程(Reader/Writer)?
}
}
================================================================
9. 临时工/合同工(ThreadPerMessage/WokerThread)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
这两种模式的基本特点,就是把工作交给其他的线程去处理,
把调用(invocation)和执行(execution)放在不同个线程中,以提高响应能力.
临时工相当灵活,呼之即来,挥之即去,不过缺点是浪费资源,不便控制.
合同工则可以弥补上述缺点,让工作持续稳定.
一般来讲,多使用线程池技术,和既定的安全框架,不多累述.来道头脑风暴吧,
====================== java : Test.java ========================
public class Test {
public void doOther(final Object obj){
new Thread(){
public void run(){
System.out.println(obj);
// Business Method
}
}.start();
}
}
================================================================
完成method方法,使下面程序只输出"12",而不输出"3";
不可以在method中print任何字符出来.
====================== java : Quest.java =======================
public class Quest {
public static void haha(Object obj){
System.out.print("1");
method(obj);
System.out.print("2");
synchronized(obj){
System.out.print("3");
}
}
public static void main(String[] args) {
haha("");
}
}
================================================================
一种答案:
==================== aes : 密码是trydofor ======================
WXDDkl4eASEwISEwIS0fccK1w6jCkCgoBcOVwpEEWmvCjBFDLRrDiwE4w7/CpSEx
NjAhZMO2w4XDicK1w4k9w4tELQ7CnyExMyFsw6dsL1Qfw7RRwqjDhcK4w49/LRjD
oRdCw7hmw47ChkPCjcODw73CqijDksKvLXIIW0TDqnbDvXbCisOhw7bCkFxrR8Ks
LS9dwpTDhgg1c8KLYyExMiHCiCkWwrs7wpotITEzIcOyw608wpd2Lj/Dt1owecOo
ITExIQPDhC3CihBpwrXCrCEzNCHCjnsSccOuwovCq8KlN8KCLXUvwpM6a3LDimXC
vWVmUsOfPDPCpC3CqsONSMKRw5N8IMKtcsKecELDq8OgwoZOLV8dw4zDpDEPwo3C
gMKkw50fVGghMTEhcjUtw455KHIdICMhMTIhfMK2ccOEccOFSMK7LRrDgcOIM1Yh
MzkhwoDDgxlpwoXCtMKww6Q3wqgtwpVzM8KSwooqw7LCv1TDi0hwc8O2wqw7LSbD
hUIRw7hxw6nDpkDDssOBYx/Dt8KQwoQtaMOfX8K6w6vDpMO/SW0Eb0fCpsKzITM0
IcKyLTDDlE3DvnbDjC7Cl8KAw7wVdiExMSE4wqN5LcOcBR7DnMO6wqlwYsOaEsKK
PhPDlMKQGC0GITE2MCHDpMK+KsOzMlvClBDDimgRFz8GLcOLHcONwrYpw7gjw7lM
V8KLwpp4ITkhVcOzLcKGQ8ODKcKSw4HDpMO2H34pF8O5GcKkOi1+VmMYwrjDrBPD
rmATGUBjFsKKwpYtITM5ISjDusOBw6fDtTDCrXXDgMKnwosBE1tyLU0RRMOvwp1M
wr3Drj/Cnz5UaHtEUi0FWCrDmTVYw6nChMKmQMKnw7NiLEXCgS0hMTMhMMOvO28r
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vcOzw5R3HcKzUX7CrE8uLcKEYxMGb8Kpw7dLITAhBQTCqsKVwpfDjHUtwoMSccKf
wo5hw6cyw70kw4pawocqwoXDrS3Dlm/CoUU4wqtXBlQVwr5hD8KTcEAtf8KmOzIh
MTMhwr3DpcKJOlfDqcK1E8KNwqPCpC3CmsKKwrzCmBZEwpIBw59TbyBZwosZw60t
X8O0wo0GwpvDozVMJX0eFMOiFm9zLcKnwp7CrkpVwo3DlMK/dcKdSijDhMOJw5oP
LQjDrhHDl8Knw58hMTAhw7Y9Mx/DtMO0wrbDjAYtwrBcw6LDiCVtworDmsK0w6fD
iMKbwqrCgHgBLcODwoTDkCExMyHDkMKnEMKTw4ojMErCjBUwwpItUsKNwr/CucOe
N1HCn1jCm8OsBmnCgwVrLcOlwqN6LmnDksKAY01vPSEzMyHCmiEwIWtMLcKhw5dU
wprCiCEzNCHDlEJLw6XDisO0wqzDvh7Diy3CvMKFITEzITM6w5rCvMOMITEwIcOK
wpjDuAM=
================================================================
10. 拿订单提货(Future)
^^^^^^^^^^^^^^^^^^^^^
要那么长时间才完成啊,我先占个座吧,好了告诉我.
Future模式,可以设置返回值,也可以采用callback,这依赖于订货者的要求.
==================== java : FutureData.java ====================
public class FutureData implements Data {
private Object data = null; // 真正返回值
private volatile boolean ready = false;
public synchronized void set(Object obj){
if(ready){
return;
}
data = obj;
ready = true;
notifyAll();
}
public synchronized Object get(){ // 一直等下去
while(!ready){
try {
wait();
} catch (InterruptedException e) {
}
}
return data;
}
public boolean isReady(){ // 可以来检查.
return ready;
}
}
================================================================
11. 你还是自裁吧(TwoPhaseTermination)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
从外界终止一个执行中的线程是不安全的,最好的办法是让它自己搞定.
不要使用Thread.stop(),它已经被 deprecated 的了,因为他强制终止线程.
==================== java : DoItYouself.java ===================
public class DoItYouself extends Thread{
private volatile boolean needShutdown = false; // 为何 volatile
public void shutdown(){
needShutdown = true;
interrupt(); // 为什么呢?
}
public final void run(){
try{
while(!needShutdown){
//doWork();
}
}catch(InterruptedException e){
}finally{
//doShutdown();
}
}
}
================================================================
另外,下面代码会输出"13",因为ShutdownHook Thread.start()会在System.exit()或
所有非Deamon线程结束时被调用.
==================== java : DoItYouself.java ===================
public class ShutdownHook{
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(
new Thread(){
public void run(){
System.out.print("3");
}
}
);
System.out.print("1");
System.exit(0);
System.out.print("2");
}
}
================================================================
[[!拓展内容, interrupt() 的作用.]]
调用interrupt(),会产生以下作用之一,不可兼得中断异常和中断状态.
1) 当线程sleep/wait/join时,抛出中断异常(InterruptedException).
2) 非1)时,线程变成中断状态(interrupted),可以使用Thread.interrupted();
======================== java : 相互转换 =======================
InterruptedException se = null;
try{
if(Thread.interrupted()){
throw new InterruptedException(); // 状态 => 异常
}
}catch(InterruptedException e){
Thread.currentThread().interrupt(); // 异常 => 状态
se = e; // 异常 => 异常
}finally{
//doShutdown();
}
if(se != null) throw e;
================================================================
12. 私有保险箱(ThreadLocalStorage)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ThreadLocal模式下的多线程行为表面上看与单线程很像,可以不用考虑互斥.
这种以线程为主键的map,可以很好的分离开线程私有的信息,就像局部变量一样.