線程同步是指對共享資源，若不是共享資源則不涉及線程同步問題。
synchronized方法可以鎖定對象，也可以鎖定代碼塊。
當鎖定對象時，修飾在類的方法前面。記住：若兩個線程同時訪問一個對象的不同方法，且對象有一個synchronized方法和一個非synchronized方法，則是兩個線程並不涉及同步問題。只有當訪問同一個對象的synchronsized方法（可以是不同的synchronized方法）時纔會有同步。
比如對象toy有如下兩個方法
package concurrent1;
public class toy {
private int number;
public toy(){
number = 100;
}
synchronized public void paint()
{
System.out.println("thread:"+Thread.currentThread().getName() + ":paint begin");
try{
int i = (int) (Math.random()*1000);
Thread.sleep(i);
}
catch(Exception e)
{
}
System.out.println("thread:"+Thread.currentThread().getName()+":paint");
System.out.println("thread:"+Thread.currentThread().getName() + ":paint end");
}
synchronized public void write()
{
System.out.println("thread:"+Thread.currentThread().getName() + ":write begin");
try{
int i = (int) (Math.random()*1000);
Thread.sleep(i);
}
catch(Exception e)
{
}
System.out.println("thread:"+Thread.currentThread().getName()+":write");
System.out.println("thread:"+Thread.currentThread().getName() + ":write end");
}
}package concurrent1;
public class Test {
public static void main(String[] args) {
// TODO Auto-generated method stub
toy mytoy = new toy();
boy myboy = new boy(mytoy);
boy youboy = new boy(mytoy);
myboy.setName("myboy");
youboy.setName("youboy");
myboy.start();
youboy.start();
}
}結果：
thread:myboy:write begin thread:myboy:write thread:myboy:write end thread:myboy:paint begin thread:myboy:paint thread:myboy:paint end thread:youboy:write begin thread:youboy:write thread:youboy:write end thread:youboy:paint begin thread:youboy:paint thread:youboy:paint end
若將paint方法前的synchronized取消掉，輸出如下，可以看出write方法是異步執行的，只有paint方法是同步執行的。
thread:myboy:write begin thread:youboy:write begin thread:myboy:write thread:myboy:write end thread:myboy:paint begin thread:myboy:paint thread:myboy:paint end thread:youboy:write thread:youboy:write end thread:youboy:paint begin thread:youboy:paint thread:youboy:paint end