Java線程安全synchronize學習
Java中，synchronized關鍵字有2種用法：java
- 做爲關鍵字修飾方法
- 修飾一個代碼塊
線程爭用
爲了探究synchronized的具體用法，能夠用一個簡單的程序來講明:dom
package fc.learn.java.synchronize;
import java.util.Random;
public class LearningSynchronized {
public enum SyncTypeTag {
OBJ_METHOD, CLASS_METHOD, KEYWARD_METHOD, NORMAL_METHOD
}
static class MyRunnable implements Runnable{
private void counting(){
for (int i = 0; i< 5; i++){
System.out.println(String.format("Thread[%s] count for [%s]", Thread.currentThread().getName(), i));
int sleepSeconds = new Random().nextInt();
sleepSeconds = sleepSeconds % 10;
sleepSeconds = sleepSeconds * sleepSeconds;
sleepSeconds = sleepSeconds % 5;
System.out.println(String.format("Thread[%s] sleep for 0.%s seconds", Thread.currentThread().getName(), sleepSeconds));
try {
Thread.sleep(sleepSeconds * 100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
private void synchronizeObjectMethod() {
synchronized (this){
counting();
}
}
private void synchronizeClasstMethod(){
synchronized (MyRunnable.class){
counting();
}
}
private synchronized void synchronizedMethod(){
counting();
}
private void normalMethod(){
counting();
}
private SyncTypeTag synType;
public MyRunnable(SyncTypeTag type){
this.synType = type;
}
@Override
public void run() {
if (this.synType == SyncTypeTag.OBJ_METHOD) {
synchronizeObjectMethod();
} else if (this.synType == SyncTypeTag.CLASS_METHOD) {
synchronizeClasstMethod();
} else if (this.synType == SyncTypeTag.KEYWARD_METHOD) {
synchronizedMethod();
} else if (this.synType == SyncTypeTag.NORMAL_METHOD) {
normalMethod();
}
}
}
public static void main(String[] args) {
Runnable r1 = new MyRunnable(SyncTypeTag.NORMAL_METHOD);
Runnable r2 = new MyRunnable(SyncTypeTag.NORMAL_METHOD);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
}
運行代碼，能夠得到結果以下：ide
Thread[Thread-1] count for [0] Thread[Thread-0] count for [0] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [1] Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [1] Thread[Thread-1] sleep for 0.0 seconds Thread[Thread-1] count for [2] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [3] Thread[Thread-0] count for [2] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [3] Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [4] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] count for [4] Thread[Thread-0] sleep for 0.1 seconds
也就是說，普通狀況下，counting()方法會被線程爭用this
類鎖 synchronized(class)
那麼咱們加鎖試試呢？首先試試對class加鎖：線程
// 以前代碼太多再也不贅述，此處只展現main方法
public static void main(String[] args) {
Runnable r1 = new MyRunnable(SyncTypeTag.CLASS_METHOD);
Runnable r2 = new MyRunnable(SyncTypeTag.CLASS_METHOD);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
能夠獲得以下結果：code
Thread[Thread-0] count for [0] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [1] Thread[Thread-0] sleep for 0.0 seconds Thread[Thread-0] count for [2] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [3] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [4] Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [0] Thread[Thread-1] sleep for 0.0 seconds Thread[Thread-1] count for [1] Thread[Thread-1] sleep for 0.0 seconds Thread[Thread-1] count for [2] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [3] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [4] Thread[Thread-1] sleep for 0.1 seconds
此時，線程再也不爭用，目的達到。orm
對象鎖 synchronized(object)
那麼試試對象鎖:htm
// 仔細觀察，這裏用的 this 也就是說，每一個線程用的不是同一把鎖
private void synchronizeObjectMethod() {
synchronized (this){
counting();
}
}
public static void main(String[] args) {
Runnable r1 = new MyRunnable(SyncTypeTag.CLASS_METHOD);
Runnable r2 = new MyRunnable(SyncTypeTag.CLASS_METHOD);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
運行代碼：對象
Thread[Thread-1] count for [0] Thread[Thread-0] count for [0] Thread[Thread-1] sleep for 0.0 seconds Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [1] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [2] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] count for [1] Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [3] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] count for [2] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [3] Thread[Thread-0] sleep for 0.0 seconds Thread[Thread-0] count for [4] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-1] count for [4] Thread[Thread-1] sleep for 0.0 seconds
能夠發現，鎖並不起做用。這個緣由也很容易理解，由於synchronizeObjectMethod方法中用的synchronized(this)進行加鎖，而咱們有2個進程對象在對counting()方法進行操做，因此會發生爭用。若是代碼修改成這樣：
// synchronizedObjectMethod修改爲這樣:
private void synchronizeObjectMethod(Object globalLock) {
synchronized (globalLock){
counting();
}
}
// 構造方法和成員變量:
private SyncTypeTag synType;
private Object globalLock;
public MyRunnable(SyncTypeTag type, Object lock){
this.synType = type;
this.globalLock = lock;
}
@Override
public void run() {
if (this.synType == SyncTypeTag.OBJ_METHOD) {
synchronizeObjectMethod(this.globalLock);
} else if (this.synType == SyncTypeTag.CLASS_METHOD) {
synchronizeClasstMethod();
} else if (this.synType == SyncTypeTag.KEYWARD_METHOD) {
synchronizedMethod();
} else if (this.synType == SyncTypeTag.NORMAL_METHOD) {
normalMethod();
}
}
// main 方法:
public static void main(String[] args) {
Object globalLock = new Object();
Runnable r1 = new MyRunnable(SyncTypeTag.OBJ_METHOD, globalLock);
Runnable r2 = new MyRunnable(SyncTypeTag.OBJ_METHOD, globalLock);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
運行代碼:
Thread[Thread-0] count for [0] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [1] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [2] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [3] Thread[Thread-0] sleep for 0.0 seconds Thread[Thread-0] count for [4] Thread[Thread-0] sleep for 0.0 seconds Thread[Thread-1] count for [0] Thread[Thread-1] sleep for 0.0 seconds Thread[Thread-1] count for [1] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-1] count for [2] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [3] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-1] count for [4] Thread[Thread-1] sleep for 0.4 seconds
爭用消失。
關鍵字 synchronized method()
接下來再試試synchronized關鍵字直接修飾方法:
public static void main(String[] args) {
Object globalLock = new Object();
Runnable r1 = new MyRunnable(SyncTypeTag.KEYWARD_METHOD, globalLock);
Runnable r2 = new MyRunnable(SyncTypeTag.KEYWARD_METHOD, globalLock);
Thread t1 = new Thread(r1);
Thread t2 = new Thread(r2);
t1.start();
t2.start();
}
運行代碼:
Thread[Thread-0] count for [0] Thread[Thread-1] count for [0] Thread[Thread-0] sleep for 0.0 seconds Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-0] count for [1] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-1] count for [1] Thread[Thread-0] count for [2] Thread[Thread-1] sleep for 0.1 seconds Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-0] count for [3] Thread[Thread-0] sleep for 0.1 seconds Thread[Thread-1] count for [2] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-0] count for [4] Thread[Thread-0] sleep for 0.4 seconds Thread[Thread-1] count for [3] Thread[Thread-1] sleep for 0.4 seconds Thread[Thread-1] count for [4] Thread[Thread-1] sleep for 0.1 seconds
依舊會起爭用，說明synchronized修飾方法，等於 synchronized(this)。
References:
http://www.cnblogs.com/GnagWang/archive/2011/02/27/1966606.html
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