Java中的死锁，乐观锁
2020-07-21 本文已影响0人
被虐的小鸡
线程的创建方式
纠正一下线程的创建方式，之前很多博客都写有三种方式，翻看了一下Thread的源码发现只有两种方式。
1.继承Thread
2.实现Runnable
* There are two ways to create a new thread of execution. One is to
* declare a class to be a subclass of <code>Thread</code>. This
* subclass should override the <code>run</code> method of class
* <code>Thread</code>. An instance of the subclass can then be
* allocated and started. For example, a thread that computes primes
* larger than a stated value could be written as follows:
* <hr><blockquote><pre>
* class PrimeThread extends Thread {
* long minPrime;
* PrimeThread(long minPrime) {
* this.minPrime = minPrime;
* }
*
* public void run() {
* // compute primes larger than minPrime
* . . .
* }
* }
* </pre></blockquote><hr>
* <p>
* The following code would then create a thread and start it running:
* <blockquote><pre>
* PrimeThread p = new PrimeThread(143);
* p.start();
* </pre></blockquote>
* <p>
* The other way to create a thread is to declare a class that
* implements the <code>Runnable</code> interface. That class then
* implements the <code>run</code> method. An instance of the class can
* then be allocated, passed as an argument when creating
* <code>Thread</code>, and started. The same example in this other
* style looks like the following:
* <hr><blockquote><pre>
* class PrimeRun implements Runnable {
* long minPrime;
* PrimeRun(long minPrime) {
* this.minPrime = minPrime;
* }
*
* public void run() {
* // compute primes larger than minPrime
* . . .
* }
* }
* </pre></blockquote><hr>
* <p>
* The following code would then create a thread and start it running:
* <blockquote><pre>
* PrimeRun p = new PrimeRun(143);
* new Thread(p).start();
* </pre></blockquote>
* <p>
至于为什么实现Callable不能算作一种方式，是因为Callable我们需要交给FutureTask来处理，而FutureTask又是Runnable的子类。
线程的状态
回顾一下线程的状态
1.可运行状态
2.运行状态
3.阻塞状态 使用synchronized加锁
4.等待状态 使用wait，sleep，join以及显示锁
5.死亡状态
死锁
死锁就是有两个线程或者两个以上线程，A线程获取了A资源之后还想获取B资源，B线程获取了B资源之后还想获取A资源，彼此都不放弃自己持有的资源。
public class Lock {
static ReentrantLock lock=new ReentrantLock();
static ReentrantLock lock1=new ReentrantLock();
static class ThreadA extends Thread{
@Override
public void run() {
super.run();
synchronized (lock){
System.out.println("lock");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1){
System.out.println("lock1");
}
}
}
}
static class ThreadB extends Thread{
@Override
public void run() {
super.run();
synchronized (lock1){
System.out.println("lock1");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock){
System.out.println("lock");
}
}
}
}
public static void main(String[] args){
new ThreadA().start();
new ThreadB().start();
}
}
死锁的条件：
1.互斥条件
2.保持持有资源
3.不剥夺已有资源
4.环路等待
解决死锁
1.设定请求锁的顺序
public class Lock {
static ReentrantLock lock=new ReentrantLock();
static ReentrantLock lock1=new ReentrantLock();
static class ThreadA extends Thread{
@Override
public void run() {
super.run();
synchronized (lock){
System.out.println("lock");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1){
System.out.println("lock1");
}
}
}
}
static class ThreadB extends Thread{
@Override
public void run() {
super.run();
synchronized (lock){
System.out.println("lock");
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock1){
System.out.println("lock1");
}
}
}
}
public static void main(String[] args){
new ThreadA().start();
new ThreadB().start();
}
}
2.使用tryLock尝试请求锁
public class Lock {
static ReentrantLock lock = new ReentrantLock();
static ReentrantLock lock1 = new ReentrantLock();
static class ThreadA extends Thread {
@Override
public void run() {
super.run();
while (true) {
if (lock.tryLock()) {
System.out.println(Thread.currentThread().getName() + "lock");
try {
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock1.tryLock()) {
try {
System.out.println(Thread.currentThread().getName() + "lock1");
break;
} finally {
lock1.unlock();
}
}
} finally {
lock.unlock();
}
}
}
}
}
static class ThreadB extends Thread {
@Override
public void run() {
super.run();
while (true) {
if (lock1.tryLock()) {
System.out.println(Thread.currentThread().getName() + "lock1");
try {
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock.tryLock()) {
try {
System.out.println(Thread.currentThread().getName() + "lock");
break;
} finally {
lock.unlock();
}
}
} finally {
lock1.unlock();
}
}
}
}
}
public static void main(String[] args) {
new ThreadA().start();
new ThreadB().start();
}
}
发现无法获取内部的锁，造成了活锁，线程还在一直运行但是无法执行内部的代码，一直在获取锁，释放锁。
解决方法：
在释放掉锁之后等待一段不同的时间
public class Lock {
static ReentrantLock lock = new ReentrantLock();
static ReentrantLock lock1 = new ReentrantLock();
static class ThreadA extends Thread {
@Override
public void run() {
super.run();
while (true) {
if (lock.tryLock()) {
System.out.println(Thread.currentThread().getName() + "lock");
try {
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock1.tryLock()) {
try {
System.out.println(Thread.currentThread().getName() + "lock1");
break;
} finally {
lock1.unlock();
}
}
} finally {
lock.unlock();
}
}
try {
sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
static class ThreadB extends Thread {
@Override
public void run() {
super.run();
while (true) {
if (lock1.tryLock()) {
System.out.println(Thread.currentThread().getName() + "lock1");
try {
try {
sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (lock.tryLock()) {
try {
System.out.println(Thread.currentThread().getName() + "lock");
break;
} finally {
lock.unlock();
}
}
} finally {
lock1.unlock();
}
}
try {
sleep(3);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
public static void main(String[] args) {
new ThreadA().start();
new ThreadB().start();
}
}
ThreadLocal
在Handler中使用了ThreadLocal用来保存Looper，使得每个线程有自己的Looper。可以起到数据隔离的作用。
内部原理：
由于每个Thread都有一个ThreadLocal.ThreadLocalMap的成员变量，ThreadLocalMap中存放了一个Entry[]，key就是ThreadLocal，value就是存放的值。数组是因为一个线程可以有多个ThreadLocal。
当ThreadLocal.set的时候，首先获取了当前线程的ThreadLocalMap。
乐观锁(CAS) compare and swap
通常我们使用的类锁，对象锁，显示锁都是悲观锁，当某一个线程拿到锁之后其他线程就无法拿到锁。
乐观锁可以保证原子操作，在对变量进行操作的时候，多个线程可以同时进入代码执行，但是会携带一个初始的值，进入CAS的时候会拿着初始值和此线程携带的初始值进行比较，如果一致那么就可以将此线程的值进行更新。如果不一致那么就重新执行代码进行计算，然后再比较....此处是一个死循环
乐观锁的问题
1.ABA
变量原始为A，线程1将变量修改为B，而线程2将变量修改成C，然后又修改成A，这个时候就出现问题了。
解决方法：加入版本戳，可以理解为一个计数器，如果初始值相同，并且计数器为0，那么就是没有被别人修改过
例如：AtomicMarkableReference(有版本戳),AtomicStampedReference(有版本戳并且可以获取被修改了几次)
2.资源开支 线程竞争比较激烈的时候，死循环
3.只能对一个变量进行原子操作 AtomicReference(将修改的变量封装成一个类，就可以对多个变量进行原子操作了)
