参考视频：多线程16：线程的优先级（狂神）
12. 线程的优先级Java提供一个线程调度器来监控程序中启动后进入就绪状态的所有线程，线程调度器按照优先级决定应该调度哪个线程来执行。线程的优先级用数字表示，范围从1~10.
Thread.MIN_PRIORITY = 1;Thread.MAX_PRIORITY = 10;Thread.NORM_PRIORITY = 5; 使用以下方式改变或获取优先级
getPriority().setPriority(int xxx) 优先级的设定建议在start()调度前。优先级低只是意味着获得调度的概率低，并不是优先级低就不会被调用了，这都是看CPU的调度。
示例一：
package com.zach.state;
//测试线程的优先级
public class TestPriority {
public static void main(String[] args) {
//主线程默认优先级
System.out.println(Thread.currentThread().getName()
+"->"
+Thread.currentThread().getPriority());
MyPriority myPriority = new MyPriority();
Thread t1 = new Thread(myPriority);
Thread t2 = new Thread(myPriority);
Thread t3 = new Thread(myPriority);
Thread t4 = new Thread(myPriority);
Thread t5 = new Thread(myPriority);
Thread t6 = new Thread(myPriority);
//先设置优先级，再启动
t1.start();
t2.setPriority(1);
t2.start();
t3.setPriority(4);
t3.start();
t4.setPriority(Thread.MAX_PRIORITY);//MAX_PRIORITY=10
t4.start();
}
}
class MyPriority implements Runnable{
@Override
public void run() {
System.out.println(Thread.currentThread().getName()
+"->"
+Thread.currentThread().getPriority());
}
}
13. 守护线程
Daemon：守护进程线程分为 用户线程 和 守护线程虚拟机必须确保用户线程执行完毕虚拟机不用等待守护线程执行完毕如，后台记录操作日志、监控内存、垃圾回收等待… …
示例一：
package com.zach.state;
//测试守护线程
//上帝守护你
public class TestDaemon {
public static void main(String[] args) {
God god = new God();
You you = new You();
Thread thread1 = new Thread(god);
thread1.setDaemon(true);//默认是false表示是用户线程，正常的线程都是用户线程。
thread1.start();//上帝守护线程启动
Thread thread2 = new Thread(you);
thread2.start();//你 用户线程启动。
}
}
//上帝
class God implements Runnable{
@Override
public void run() {
while (true){
System.out.println("God Bless U!");
}
}
}
//你
class You implements Runnable{
@Override
public void run() {
System.out.println("===== Hello World! =====");
for (int i = 0; i <= 3650; i++) {
System.out.println("你在修仙之路的第"+i+"天");
}
System.out.println("===== Goodbye World! =====");
}
}
14. 线程同步
线程同步发生在多个线程操作同一个资源时。并发问题：多个线程访问同一个对象。
线程同步由于同一进程的多个线程共享同一块存储空间，在带来方便的同时，也带来了访问冲突问题，为了保证数据在方法中被访问时的正确性，在访问时加入锁机制synchronized ，当一个线程获得对象的排它锁，独占资源，其他线程必须等待，使用后释放锁即可。存在以下问题 ：
一个线程持有锁会导致其他所有需要此锁的线程挂起；在多线程竞争下，加锁、释放锁会导致比较多的 上下文切换 和 调度延时 ，从而引起性能问题；如果一个优先级高的线程等待一个优先级低的线程释放锁会导致优先级倒置，从而引起性能问题。 15. 三大不安全案例
示例一：
package com.zach.syn;
//不安全的买票
//线程不安全，有0 有负数 有重复
public class UnsafeBuyTicket {
public static void main(String[] args) {
BuyTicket station = new BuyTicket();
new Thread(station,"黄牛").start();
new Thread(station,"退钱哥").start();
new Thread(station,"夺命书生").start();
}
}
class BuyTicket implements Runnable{
//票
private int ticketNums = 10;
boolean flag = true;//外部停止方式
@Override
public void run() {
//买票
while (flag){
buy();
}
}
private void buy() {
//判断是否有票
if (ticketNums<=0){
flag = false;
return;
}
//模拟延时（放大问题的发生性）
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
//买票
System.out.println(Thread.currentThread().getName()+"拿到第"+ticketNums--+"张票");
}
}
示例二：
package com.zach.syn;
//不安全的取钱
//两个人去银行取钱，账户
public class UnsafeBank {
public static void main(String[] args) {
//账户
Account account = new Account(100, "Dreamer Fund");
Drawing zach = new Drawing(account,50,"柴客");
Drawing dreamer = new Drawing(account,100,"鸡妈");
zach.start();
dreamer.start();
}
}
//账户
class Account{
int money;//余额
String name;//卡名
public Account(int money, String name) {
this.money = money;
this.name = name;
}
}
//银行：模拟取款
class Drawing extends Thread{
Account account;//账户
int drawingMoney;//取了多少钱
int nowMoney;//现在有多少线
public Drawing(Account account, int drawingMoney, String name) {
super(name);
this.account = account;
this.drawingMoney = drawingMoney;
}
//取钱
@Override
public void run() {
//判断有没有钱
if (account.money-drawingMoney<0){
System.out.println(Thread.currentThread().getName()+"钱不够，取不了");
return;
}
try {
Thread.sleep(1000);//方法问题发生性
} catch (InterruptedException e) {
e.printStackTrace();
}
//卡内余额 = 余额 - 你取的钱
account.money -= drawingMoney;
//你手里的钱
nowMoney += drawingMoney;
System.out.println(account.name+"余额为："+ account.money);
// Thread.currentThread().getName() = this.getName()
System.out.println(this.getName()+"手里的钱"+nowMoney);
}
}
示例三：
package com.zach.syn;
import java.util.ArrayList;
//线程不安全的集合
public class UnsafeList {
public static void main(String[] args) {
ArrayList list = new ArrayList();
for (int i = 0; i < 10000; i++) {
new Thread(()->{
list.add(Thread.currentThread().getName());
}).start();
}
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());
}
}
16. 同步方法
同步方法的弊端
方法里面需要修改的内容才需要锁，锁得太多，浪费资源。
示例一：
package com.zach.syn;
//不安全的买票
//线程不安全，有0 有负数 有重复
public class UnsafeBuyTicket {
public static void main(String[] args) {
BuyTicket station = new BuyTicket();
new Thread(station,"黄牛").start();
new Thread(station,"退钱哥").start();
new Thread(station,"夺命书生").start();
}
}
class BuyTicket implements Runnable{
//票
private int ticketNums = 10;
boolean flag = true;//外部停止方式
@Override
public void run() {
//买票
while (flag){
buy();
}
}
//synchronized 同步方法，锁的是this
private synchronized void buy() {
//判断是否有票
if (ticketNums<=0){
flag = false;
return;
}
//模拟延时（放大问题的发生性）
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
//买票
System.out.println(Thread.currentThread().getName()+"拿到第"+ticketNums--+"张票");
}
}
示例二：
package com.zach.syn;
//不安全的取钱
//两个人去银行取钱，账户
public class UnsafeBank {
public static void main(String[] args) {
//账户
Account account = new Account(100, "Dreamer Fund");
Drawing zach = new Drawing(account,50,"柴客");
Drawing dreamer = new Drawing(account,100,"鸡妈");
zach.start();
dreamer.start();
}
}
//账户
class Account{
int money;//余额
String name;//卡名
public Account(int money, String name) {
this.money = money;
this.name = name;
}
}
//银行：模拟取款
class Drawing extends Thread{
Account account;//账户
int drawingMoney;//取了多少钱
int nowMoney;//现在有多少线
public Drawing(Account account, int drawingMoney, String name) {
super(name);
this.account = account;
this.drawingMoney = drawingMoney;
}
//取钱
//synchronized 默认锁的是this.
@Override
public void run() {
//锁住的对象是变化的量，即需要增删改的对象：这里是account，而不是this。
synchronized (account){
//判断有没有钱
if (account.money-drawingMoney<0){
System.out.println(Thread.currentThread().getName()+"钱不够，取不了");
return;
}
try {
Thread.sleep(1000);//方法问题发生性
} catch (InterruptedException e) {
e.printStackTrace();
}
//卡内余额 = 余额 - 你取的钱
account.money -= drawingMoney;
//你手里的钱
nowMoney += drawingMoney;
System.out.println(account.name+"余额为："+ account.money);
// Thread.currentThread().getName() = this.getName()
System.out.println(this.getName()+"手里的钱"+nowMoney);
}
}
}
示例三：
package com.zach.syn;
import java.util.ArrayList;
//线程不安全的集合
public class UnsafeList {
public static void main(String[] args) {
ArrayList list = new ArrayList();
for (int i = 0; i < 10000; i++) {
new Thread(()->{
synchronized (list){
list.add(Thread.currentThread().getName());
}
}).start();
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());
}
}
17. CopyonWriteArrayList
示例一：
package com.zach.syn;
import java.util.concurrent.CopyOnWriteArrayList;//并发包;concurrent（同时）
//测试JUC安全类型的集合
public class TestJUC {
public static void main(String[] args) {
CopyOnWriteArrayList list = new CopyOnWriteArrayList();
for (int i = 0; i < 10000; i++) {
new Thread(()->{
list.add(Thread.currentThread().getName());
}).start();
try {
Thread.sleep(1);//如果这里不睡眠 外面睡眠1ms还是出问题
} catch (InterruptedException e) {
e.printStackTrace();
}
}
try {
Thread.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(list.size());
}
}
