架构师养成记--3.synchronized细节问题

一、synchronized有锁重入的特点,某个线程得到对象的锁后,再次请求此对象可以再次得到改对象的锁。如下示例,在method1中调用method2,在method2中调用method3,而method1、method2和method3都是加了synchronized关键字的。

 /**
* synchronized的重入
* @author alienware
*
*/
public class SyncDubbo1 { public synchronized void method1(){
System.out.println("method1..");
method2();
}
public synchronized void method2(){
System.out.println("method2..");
method3();
}
public synchronized void method3(){
System.out.println("method3..");
} public static void main(String[] args) {
final SyncDubbo1 sd = new SyncDubbo1();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
sd.method1();
}
});
t1.start();
}
}

二、父类和子类的方法都是synchronized的,在子类的方法中调用父类的方法,也是线程安全的。

 /**
* synchronized的重入
* @author alienware
*
*/
public class SyncDubbo2 { static class Main {
public int i = 10;
public synchronized void operationSup(){
try {
i--;
System.out.println("Main print i = " + i);
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} static class Sub extends Main {
public synchronized void operationSub(){
try {
while(i > 0) {
i--;
System.out.println("Sub print i = " + i);
Thread.sleep(100);
this.operationSup();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
Sub sub = new Sub();
sub.operationSub();
}
}); t1.start();
} }

执行结果:

架构师养成记--3.synchronized细节问题

三、synchronized方法内抛异常怎么处理

  throw RuntimeException打断此线程或者记录日志然后continue,选择哪种方案取决于具体业务要求。

 /**
* synchronized异常
* @author alienware
*
*/
public class SyncException { private int i = 0;
public synchronized void operation(){
while(true){
try {
i++;
Thread.sleep(100);
System.out.println(Thread.currentThread().getName() + " , i = " + i);
if(i == 20){
//Integer.parseInt("a");
throw new RuntimeException();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final SyncException se = new SyncException();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
se.operation();
}
},"t1");
t1.start();
} }

四、synchronized代码块锁,实用起来也会比较灵活

  this、class、Object都可以用来作为代码块锁

 /**
* 使用synchronized代码块加锁,比较灵活
* @author alienware
*
*/
public class ObjectLock { public void method1(){
synchronized (this) { //对象锁
try {
System.out.println("do method1..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public void method2(){ //类锁
synchronized (ObjectLock.class) {
try {
System.out.println("do method2..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} private Object lock = new Object();
public void method3(){ //任何对象锁
synchronized (lock) {
try {
System.out.println("do method3..");
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) { final ObjectLock objLock = new ObjectLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method1();
}
});
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method2();
}
});
Thread t3 = new Thread(new Runnable() {
@Override
public void run() {
objLock.method3();
}
}); t1.start();
t2.start();
t3.start(); } }

五、尽量不要用String常量作为锁

如下代码只会有t1线程运行,但可以new一个String对象。

 /**
* synchronized代码块对字符串的锁,注意String常量池的缓存功能
* @author alienware
*
*/
public class StringLock { public void method() {
//new String("字符串常量")
synchronized ("字符串常量") {
try {
while(true){
System.out.println("当前线程 : " + Thread.currentThread().getName() + "开始");
Thread.sleep(1000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + "结束");
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
} public static void main(String[] args) {
final StringLock stringLock = new StringLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
stringLock.method();
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
stringLock.method();
}
},"t2"); t1.start();
t2.start();
}
}

六、一个对象被用作锁时,这个对象内的属性发生变化不会影响锁的使用。

 /**
* 同一对象属性的修改不会影响锁的情况
* @author alienware
*
*/
public class ModifyLock { private String name ;
private int age ; public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
} public synchronized void changeAttributte(String name, int age) {
try {
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 开始");
this.setName(name);
this.setAge(age); System.out.println("当前线程 : " + Thread.currentThread().getName() + " 修改对象内容为: "
+ this.getName() + ", " + this.getAge()); Thread.sleep(2000);
System.out.println("当前线程 : " + Thread.currentThread().getName() + " 结束");
} catch (InterruptedException e) {
e.printStackTrace();
}
} public static void main(String[] args) {
final ModifyLock modifyLock = new ModifyLock();
Thread t1 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("张三", 20);
}
},"t1");
Thread t2 = new Thread(new Runnable() {
@Override
public void run() {
modifyLock.changeAttributte("李四", 21);
}
},"t2"); t1.start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
t2.start();
} }
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