1、什么是JUC
源码+官方文档
JUC是 java util concurrent
面试高频问JUC~!
java.util 是Java的一个工具包~
业务:普通的线程代码 Thread
Runnable: 没有返回值、效率相比于Callable 相对较低!
2、线程和进程
进程:一个程序,QQ.EXE Music.EXE;数据+代码+pcb
一个进程可以包含多个线程,至少包含一个线程!
Java默认有几个线程?2个线程! main线程、GC线程
线程:开了一个进程Typora,写字,等待几分钟会进行自动保存(线程负责的)
对于Java而言:Thread、Runable、Callable进行开启线程的,我们之前。
提问?JAVA真的可以开启线程吗? 开不了的!
public synchronized void start() {
/**
* This method is not invoked for the main method thread or "system"
* group threads created/set up by the VM. Any new functionality added
* to this method in the future may have to also be added to the VM.
*
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented. */
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
//这是一个C++底层,Java是没有权限操作底层硬件的
private native void start0();
Java是没有权限去开启线程、操作硬件的,这是一个native的一个本地方法,它调用的底层的C++代码。
并发、并行
并发: 多线程操作同一个资源。
- CPU 只有一核,模拟出来多条线程,天下武功,唯快不破。那么我们就可以使用CPU快速交替,来模拟多线程。
并行: 多个人一起行走
- CPU多核,多个线程可以同时执行。 我们可以使用线程池!
public class Test1 {
public static void main(String[] args) {
//获取cpu的核数
System.out.println(Runtime.getRuntime().availableProcessors());
}
}
并发编程的本质:充分利用CPU的资源!
线程有几个状态?
线程的状态:6个状态
public enum State {
/**
* Thread state for a thread which has not yet started.
*/
//新生
NEW,
/**
* Thread state for a runnable thread. A thread in the runnable
* state is executing in the Java virtual machine but it may
* be waiting for other resources from the operating system
* such as processor.
*/
//运行
RUNNABLE,
/**
* Thread state for a thread blocked waiting for a monitor lock.
* A thread in the blocked state is waiting for a monitor lock
* to enter a synchronized block/method or
* reenter a synchronized block/method after calling
* {@link Object#wait() Object.wait}.
*/
//阻塞
BLOCKED,
/**
* Thread state for a waiting thread.
* A thread is in the waiting state due to calling one of the
* following methods:
* <ul>
* <li>{@link Object#wait() Object.wait} with no timeout</li>
* <li>{@link #join() Thread.join} with no timeout</li>
* <li>{@link LockSupport#park() LockSupport.park}</li>
* </ul>
*
* <p>A thread in the waiting state is waiting for another thread to
* perform a particular action.
*
* For example, a thread that has called <tt>Object.wait()</tt>
* on an object is waiting for another thread to call
* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
* that object. A thread that has called <tt>Thread.join()</tt>
* is waiting for a specified thread to terminate.
*/
//等待
WAITING,
/**
* Thread state for a waiting thread with a specified waiting time.
* A thread is in the timed waiting state due to calling one of
* the following methods with a specified positive waiting time:
* <ul>
* <li>{@link #sleep Thread.sleep}</li>
* <li>{@link Object#wait(long) Object.wait} with timeout</li>
* <li>{@link #join(long) Thread.join} with timeout</li>
* <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
* <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
* </ul>
*/
//超时等待
TIMED_WAITING,
/**
* Thread state for a terminated thread.
* The thread has completed execution.
*/
//终止
TERMINATED;
}
wait/sleep的区别
1、来自不同的类
wait => Object
sleep => Thread
一般情况企业中使用休眠是:
TimeUnit.DAYS.sleep(1); //休眠1天
TimeUnit.SECONDS.sleep(1); //休眠1s
2、关于锁的释放
wait 会释放锁;
sleep睡觉了,不会释放锁;
3、使用的范围是不同的
wait 必须在同步代码块中;
sleep 可以在任何地方睡;
4、是否需要捕获异常
wait是不需要捕获异常;
sleep必须要捕获异常;
3、Lock锁(重点)
3、Lock锁(重点)
传统的Synchronized
/**
* 真正的多线程开发
* 线程就是一个单独的资源类,没有任何的附属操作!
*/
public class SaleTicketDemo01 {
public static void main(String[] args) {
//多线程操作
//并发:多线程操作同一个资源类,把资源类丢入线程
Ticket ticket = new Ticket();
//@FunctionalInterface 函数式接口 jdk1.8之后 lambda表达式
new Thread(()->{
for(int i=0;i<40;i++){
ticket.sale();
}
},"A").start();
new Thread(()->{
for(int i=0;i<40;i++){
ticket.sale();
}
},"B").start();
new Thread(()->{
for(int i=0;i<40;i++){
ticket.sale();
}
},"C").start();
}
}
//资源类
//属性+方法
//oop
class Ticket{
private int number=50;
//卖票的方式
// synchronized 本质:队列,锁
public synchronized void sale(){
if(number>0){
System.out.println(Thread.currentThread().getName()+" 卖出了第"+number+" 张票,剩余:"+number+" 张票");
number--;
}
}
}
Lock接口
公平锁: 十分公平,必须先来后到~;
非公平锁: 十分不公平,可以插队;(默认为非公平锁)
public class SaleTicketDemo02 {
public static void main(String[] args) {
//多线程操作
//并发:多线程操作同一个资源类,把资源类丢入线程
Ticket2 ticket = new Ticket2();
new Thread(()->{for(int i=0;i<40;i++) ticket.sale(); },"A").start();
new Thread(()->{for(int i=0;i<40;i++) ticket.sale(); },"B").start();
new Thread(()->{for(int i=0;i<40;i++) ticket.sale(); },"C").start();
}
}
//lock三部曲
//1、 Lock lock=new ReentrantLock();
//2、 lock.lock() 加锁
//3、 finally=> 解锁:lock.unlock();
class Ticket2{
private int number=50;
Lock lock=new ReentrantLock();
//卖票的方式
// 使用Lock 锁
public void sale(){
//加锁
lock.lock();
try {
//业务代码
if(number>=0){
System.out.println(Thread.currentThread().getName()+" 卖出了第"+number+" 张票,剩余:"+number+" 张票");
number--;
}
}catch (Exception e) {
e.printStackTrace();
}
finally {
//解锁
lock.unlock();
}
}
}
Synchronized 和 Lock区别
1、Synchronized 内置的Java关键字,Lock是一个Java类
-
2、Synchronized 无法判断获取锁的状态,Lock可以判断
-
3、Synchronized 会自动释放锁,lock必须要手动加锁和手动释放锁!可能会遇到死锁
-
4、Synchronized 线程1(获得锁->阻塞)、线程2(等待);
lock就不一定会一直等待下去,lock会有一个trylock去尝试获取锁,不会造成长久的等待。
5、Synchronized 是可重入锁,不可以中断的,非公平的;Lock,可重入的,可以判断锁,可以自己设置公平锁和非公平锁;
6、Synchronized 适合锁少量的代码同步问题,Lock适合锁大量的同步代码;
锁到底是什么? 如何判断锁的是谁?
4、生产者和消费者问题!
Synchronized wait notify可以实现,该方法是传统版本;
我们这次使用lock版本
Synchronized版本
public class A {
public static void main(String[] args) {
Data data = new Data();
new Thread(()->{for(int i=0;i<10;i++) {
try {
data.increment();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
},"A").start();
new Thread(()->{for(int i=0;i<10;i++) {
try {
data.decrement();
} catch (InterruptedException e) {
e.printStackTrace();
}
}},"B").start();
}
}
class Data{
//数字 资源类
private int number = 0;
//+1
public synchronized void increment() throws InterruptedException {
if(number!=0){
//等待操作
this.wait();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
//通知其他线程 我+1完毕了
this.notifyAll();
}
//-1
public synchronized void decrement() throws InterruptedException {
if(number==0){
//等待操作
this.wait();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
//通知其他线程 我-1完毕了
this.notifyAll();
}
}
问题存在,A线程B线程,现在如果我有四个线程A B C D!
解决方案: if 改为while即可,防止虚假唤醒
JUC版本的生产者和消费者问题
await、signal 替换 wait、notify
通过Lock找到Condition
public class B {
public static void main(String[] args) {
Data2 data = new Data2();
new Thread(()->{for(int i=0;i<10;i++) {
data.increment();
}
},"A").start();
new Thread(()->{for(int i=0;i<10;i++) {
data.decrement();
}},"B").start();
new Thread(()->{for(int i=0;i<10;i++) {
data.increment();
}
},"C").start();
new Thread(()->{for(int i=0;i<10;i++) {
data.decrement();
}
},"D").start();
}
}
class Data2{
//数字 资源类
private int number = 0;
//lock锁
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
//+1
public void increment() {
lock.lock();
try{
//业务
while (number!=0){
//等待操作
condition.await();
}
number++;
System.out.println(Thread.currentThread().getName()+"=>"+number);
//通知其他线程 我+1完毕了
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
//-1
public void decrement() {
lock.lock();
try{
//业务
while (number==0){
//等待操作
condition.await();
}
number--;
System.out.println(Thread.currentThread().getName()+"=>"+number);
//通知其他线程 我+1完毕了
condition.signalAll();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
Condition的优势:精准的通知和唤醒的线程!
如果我们要指定通知的下一个进行顺序怎么办呢? 我们可以使用Condition来指定通知进程~
/**
* A 执行完 调用B
* B 执行完 调用C
* C 执行完 调用A
*/
public class C {
public static void main(String[] args) {
Data3 data3 = new Data3();
new Thread(()->{
for(int i=0;i<10;i++){
data3.printA();
}
},"A").start();
new Thread(()->{
for(int i=0;i<10;i++){
data3.printB();
}
},"B").start();
new Thread(()->{
for(int i=0;i<10;i++){
data3.printC();
}
},"C").start();
}
}
class Data3{
//资源类
private Lock lock=new ReentrantLock();
private Condition condition1 = lock.newCondition();
private Condition condition2 = lock.newCondition();
private Condition condition3 = lock.newCondition();
private int number = 1; //1A 2B 3C
public void printA(){
lock.lock();
try {
//业务 判断 -> 执行 -> 通知
while(number!=1){
//等待
condition1.await();
}
//操作
System.out.println(Thread.currentThread().getName()+",AAAAA");
//唤醒指定的线程
number=2;
condition2.signal(); // 唤醒2
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void printB(){
lock.lock();
try {
//业务 判断 -> 执行 -> 通知
while (number!=2){
condition2.await();
}
System.out.println(Thread.currentThread().getName()+",BBBBB");
//唤醒3
number=3;
condition3.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
public void printC(){
lock.lock();
try {
//业务 判断 -> 执行 -> 通知
while(number!=3){
condition3.await();
}
System.out.println(Thread.currentThread().getName()+",CCCCC");
//唤醒1
number=1;
condition1.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}