synchronized 实例

synchronized? Java语言的关键字，当它用来修饰一个方法或者一个代码块的时候，能够保证在同一时刻最多只有一个线程执行该段代码。

先看几个例子：

?

?实例一：

import java.util.ArrayList;
import java.util.List;

public class MyThread {
?public void printNum(List list) {
??synchronized (list) {
???if (null != list) {
????for (int i = 0; i < list.size(); i++) {
?????System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
?????try {
??????Thread.sleep(100);
?????} catch (InterruptedException e) {
??????e.printStackTrace();
?????}
????}
???}
??}
?}

?public static List getList() {
??List list = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list.add(i);
??}
??return list;
?}

?public static void main(String[] args) {
??final MyThread mt = new MyThread();
??final MyThread mt1 = new MyThread();
??final List list = mt.getList();
??
??Thread t1 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list);
???}
??}, "t1");
??
??Thread t2 = new Thread(new Runnable(){
???public void run() {
????mt1.printNum(list);
???}
??}, "t2");
??t1.start();
??t2.start();

?}

}

输出结果：

?t1:1
t1:2
t1:3
t1:4
t1:5
t2:1
t2:2
t2:3
t2:4
t2:5

?

?

实例二：

? import java.util.ArrayList;
import java.util.List;

public class MyThread {
?public void printNum(List list) {
??synchronized (list) {
???if (null != list) {
????for (int i = 0; i < list.size(); i++) {
?????System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
?????try {
??????Thread.sleep(100);
?????} catch (InterruptedException e) {
??????e.printStackTrace();
?????}
????}
???}
??}
?}

?public static List getList() {
??List list = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list.add(i);
??}
??return list;
?}
???
?public static void main(String[] args) {
??final MyThread mt = new MyThread();
??final MyThread mt1 = new MyThread();
??final List list = mt.getList();
??final List list1 = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list1.add(i);
??}
??Thread t1 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list);
???}
??}, "t1");
??
??Thread t2 = new Thread(new Runnable(){
???public void run() {
????mt1.printNum(list1);
???}
??}, "t2");
??t1.start();
??t2.start();

?}

}

?

输出结果：

t1:1
t2:1
t1:2
t2:2
t2:3
t1:3
t1:4
t2:4
t2:5
t1:5

?

?实例三：

?

? import java.util.ArrayList;
import java.util.List;

public class MyThread {
?public? void printNum(List list) {
??synchronized (this) {
???if (null != list) {
????for (int i = 0; i < list.size(); i++) {
?????System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
?????try {
??????Thread.sleep(100);
?????} catch (InterruptedException e) {
??????e.printStackTrace();
?????}
????}
???}
??}
?}

?public static List getList() {
??List list = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list.add(i);
??}
??return list;
?}
???
?public static void main(String[] args) {
??final MyThread mt = new MyThread();
??final MyThread mt1 = new MyThread();
??final List list = mt.getList();
??final List list1 = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list1.add(i);
??}
??Thread t1 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list);
???}
??}, "t1");
??
??Thread t2 = new Thread(new Runnable(){
???public void run() {
????mt1.printNum(list1);
???}
??}, "t2");
??t1.start();
??t2.start();

?}

}

?

输出结果：

?t1:1
t2:1
t1:2
t2:2
t1:3
t2:3
t1:4
t2:4
t1:5
t2:5

?

实例四：

? import java.util.ArrayList;
import java.util.List;

public class MyThread {
?public? void printNum(List list) {
??synchronized (this) {
???if (null != list) {
????for (int i = 0; i < list.size(); i++) {
?????System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
?????try {
??????Thread.sleep(100);
?????} catch (InterruptedException e) {
??????e.printStackTrace();
?????}
????}
???}
??}
?}

?public static List getList() {
??List list = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list.add(i);
??}
??return list;
?}
???
?public static void main(String[] args) {
??final MyThread mt = new MyThread();
??final MyThread mt1 = new MyThread();
??final List list = mt.getList();
??final List list1 = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list1.add(i);
??}
??Thread t1 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list);
???}
??}, "t1");
??
??Thread t2 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list1);
???}
??}, "t2");
??t1.start();
??t2.start();

?}

}

?

输出结果：

?t1:1
t1:2
t1:3
t1:4
t1:5
t2:1
t2:2
t2:3
t2:4
t2:5

?

实例五：

?public class MyThread {
?public? void printNum(List list) {
??synchronized (list) {
???if (null != list) {
????for (int i = 0; i < list.size(); i++) {
?????System.out.println(Thread.currentThread().getName() + ":"+ list.get(i));
?????try {
??????Thread.sleep(100);
?????} catch (InterruptedException e) {
??????e.printStackTrace();
?????}
????}
???}
??}
?}

?public static List getList() {
??List list = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list.add(i);
??}
??return list;
?}
???
?public static void main(String[] args) {
??final MyThread mt = new MyThread();
??final MyThread mt1 = new MyThread();
??final List list = mt.getList();
??final List list1 = new ArrayList();
??for (int i = 1; i <= 5; i++) {
???list1.add(i);
??}
??Thread t1 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list);
???}
??}, "t1");
??
??Thread t2 = new Thread(new Runnable(){
???public void run() {
????mt.printNum(list1);
???}
??}, "t2");
??t1.start();
??t2.start();

?}

}

输出结果：

t1:1
t2:1
t1:2
t2:2
t2:3
t1:3
t1:4
t2:4
t2:5
t1:5

?

由以上实例可以得出，不同的线程调用的必须是同一个对象，并且该对象与synchronized 锁的对象是同一个类时，才能够保证在同一时刻最多只有一个线程执行该段代码。