java 同步和异步
我想的有点杂，先写在说。
1，关于线程安全，如果一个方法里面全部是局部变量，那么是没有必要同步的，也就是说它是线程安全的。也就是不必加synchronized，servelet里面的的HttpServlet就是这样， PrintWriter out = response.getWriter();这句话中这个out流本来可以作为HttpServlet的类变量的，但是如果是那样就需要同步了，影响效率呀。
2，关于锁，
class Test{
public synchronized void f1(){
//do something here
}
public void f2(){
synchronized(this){
//do something here
}
}
}
上面的f1()和f2()效果一致, synchronized取得的锁都是Test某个实列(this)的锁.
比如: Test t = new Test();
线程A调用t.f2()时, 线程B无法进入t.f1(),直到t.f2()结束.
作用: 多线程中访问Test的同一个实例的同步方法时会进行同步.
也就是说在f1()前面加上synchronized ，那线程在调用这个方法的时候取得了这个对象的锁，也就是t的锁，
其他的线程想访问t的其他同步方法,比如f2()就不行，其实这样是不好的，很多时候我们用另外实例变量来锁。
class Test{
private static final Object obj=new Object();
public synchronized void f1(){
//do something here
}
public void f2(){
synchronized(obj){
//do something here
}
}
}
此时，如果一个线程访问f2，它拿到的是obj对象的锁，别的线程依然可以访问f1().
3,原子性，先看代码：
package com.test;
public class Test extends Thread {
private int a = 0;
private int b = 0;
public static void main(String[] args) {
Test test = new Test();
for (int i = 0; i < 10; i++) {
Thread thread = new Thread(test, "thread-" + i);
thread.start();
}
}
public synchronized void doWrite() {
a++;
try {
sleep((int)(Math.random()*100));
}
catch (Exception e) {
}
b++;
try {
sleep((int)(Math.random()*100));
}
catch (Exception e) {
}
}
public synchronized void print() {
System.out.println("" + Thread.currentThread().getName() + ":a:" + a);
System.out.println("" + Thread.currentThread().getName() + ":b:" + b);
}
public void run() {
f();
}
public synchronized void f() {
for (int i = 0; i < 5; i++) {
doWrite();
print();
}
}
}
输出结果：
thread-0:a:1 thread-0:b:1 thread-0:a:2 thread-0:b:2 thread-0:a:3 thread-0:b:3 thread-0:a:4 thread-0:b:4 thread-0:a:5 thread-0:b:5 thread-1:a:6 thread-1:b:6 thread-1:a:7 thread-1:b:7 thread-1:a:8 thread-1:b:8 thread-1:a:9 thread-1:b:9 thread-1:a:10 thread-1:b:10 thread-2:a:11 thread-2:b:11 thread-2:a:12 thread-2:b:12 thread-2:a:13 thread-2:b:13 thread-2:a:14 thread-2:b:14 thread-2:a:15 thread-2:b:15 thread-3:a:16 thread-3:b:16 thread-3:a:17 thread-3:b:17 thread-3:a:18 thread-3:b:18 thread-3:a:19 thread-3:b:19 thread-3:a:20 thread-3:b:20 thread-4:a:21 thread-4:b:21 thread-4:a:22 thread-4:b:22 thread-4:a:23 thread-4:b:23 thread-4:a:24 thread-4:b:24 thread-4:a:25 thread-4:b:25 thread-5:a:26 thread-5:b:26 thread-5:a:27 thread-5:b:27 thread-5:a:28 thread-5:b:28 thread-5:a:29 thread-5:b:29 thread-5:a:30 thread-5:b:30 thread-6:a:31 thread-6:b:31 thread-6:a:32 thread-6:b:32 thread-6:a:33 thread-6:b:33 thread-6:a:34 thread-6:b:34 thread-6:a:35 thread-6:b:35 thread-7:a:36 thread-7:b:36 thread-7:a:37 thread-7:b:37 thread-7:a:38 thread-7:b:38 thread-7:a:39 thread-7:b:39 thread-7:a:40 thread-7:b:40 thread-8:a:41 thread-8:b:41 thread-8:a:42 thread-8:b:42 thread-8:a:43 thread-8:b:43 thread-8:a:44 thread-8:b:44 thread-8:a:45 thread-8:b:45 thread-9:a:46 thread-9:b:46 thread-9:a:47 thread-9:b:47 thread-9:a:48 thread-9:b:48 thread-9:a:49 thread-9:b:49 thread-9:a:50 thread-9:b:50
我们可以看到是很标准的输出。
我们在run方法里面改点东西：
package com.test;
public class Test extends Thread {
private int a = 0;
private int b = 0;
public static void main(String[] args) {
Test test = new Test();
for (int i = 0; i < 10; i++) {
Thread thread = new Thread(test, "thread-" + i);
thread.start();
}
}
public synchronized void doWrite() {
a++;
try {
sleep((int)(Math.random()*100));
}
catch (Exception e) {
}
b++;
try {
sleep((int)(Math.random()*100));
}
catch (Exception e) {
}
}
public synchronized void print() {
System.out.println("" + Thread.currentThread().getName() + ":a:" + a);
System.out.println("" + Thread.currentThread().getName() + ":b:" + b);
}
public void run() {
for (int i = 0; i < 5; i++) {
doWrite();
print();
}
}
}
输出的结果就不确定了：
thread-0:a:10 thread-0:b:10 thread-1:a:10 thread-1:b:10 thread-2:a:10 thread-2:b:10 thread-3:a:10 thread-3:b:10 thread-4:a:10 thread-4:b:10 thread-5:a:10 thread-5:b:10 thread-6:a:10 thread-6:b:10 thread-7:a:10 thread-7:b:10 thread-8:a:10 thread-8:b:10 thread-9:a:10 thread-9:b:10 thread-0:a:20 thread-0:b:20 thread-1:a:20 thread-1:b:20 thread-2:a:20 thread-2:b:20 thread-3:a:20 thread-3:b:20 thread-4:a:20 thread-4:b:20 thread-5:a:20 thread-5:b:20 thread-6:a:20 thread-6:b:20 thread-7:a:20 thread-7:b:20 thread-8:a:20 thread-8:b:20 thread-9:a:20 thread-9:b:20 thread-0:a:30 thread-0:b:30 thread-1:a:30 thread-1:b:30 thread-2:a:30 thread-2:b:30 thread-3:a:30 thread-3:b:30 thread-4:a:30 thread-4:b:30 thread-5:a:30 thread-5:b:30 thread-6:a:30 thread-6:b:30 thread-7:a:30 thread-7:b:30 thread-8:a:30 thread-8:b:30 thread-9:a:30 thread-9:b:30 thread-0:a:40 thread-0:b:40 thread-1:a:40 thread-1:b:40 thread-2:a:40 thread-2:b:40 thread-3:a:40 thread-3:b:40 thread-4:a:40 thread-4:b:40 thread-5:a:40 thread-5:b:40 thread-6:a:40 thread-6:b:40 thread-7:a:40 thread-7:b:40 thread-8:a:40 thread-8:b:40 thread-9:a:40 thread-9:b:40 thread-0:a:50 thread-0:b:50 thread-1:a:50 thread-1:b:50 thread-2:a:50 thread-2:b:50 thread-3:a:50 thread-3:b:50 thread-4:a:50 thread-4:b:50 thread-5:a:50 thread-5:b:50 thread-6:a:50 thread-6:b:50 thread-7:a:50 thread-7:b:50 thread-8:a:50 thread-8:b:50 thread-9:a:50 thread-9:b:50
我们的目的就是要 完数据后在输出，所以应该确保这个过程的原子性。直接写在run方法里面就没能确保原子性。
先写到这，以后在加吧。