什么是线程死锁？如何解决死锁问题

死锁，一组互相竞争的资源的线程之间相互等待，导致永久阻塞的现象。

如下图所示：

与死锁对应的，还有活锁，是指线程没有出现阻塞，但是无限循环。

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有一个经典的银行转账例子如下：

我们有个账户类，其中有两个属性：账户名和余额。
它有两个方法：转入、转出。

public class Account {private String countName;private int balance;public Account(String countName, int balance) {this.countName = countName;this.balance = balance;}/*** 转出金额，更新转出方余额，金额减少* @param amount*/public void debit(int amount){this.balance -= amount;}/*** 存入金额，更新转入方余额,金额增多* @param amount*/public void credit(int amount){this.balance += amount;}public String getCountName() {return countName;}public void setCountName(String countName) {this.countName = countName;}public int getBalance() {return balance;}public void setBalance(int balance) {this.balance = balance;}
}

还有一个转账操作类，它有三个属性：转入账户、转出账户、转账金额。
它实现了Runnable接口，run方法中是转账逻辑代码。
我们使用 while(true) 让他不停的进行转账操作：如果账户余额大于转账金额，就让转出账号减少amount，转入账户增加amount。打印出线程名、金额转移方向、以及每个账户余额。
我们在main方法中进行测试，创建两个转账账户，使用两个线程操作这两个账户，让他们相互转账。

public class TransferAccount implements Runnable{private Account fromAccount;private Account toAccount;private int amount;public TransferAccount(Account fromAccount, Account toAccount, int amount) {this.fromAccount = fromAccount;this.toAccount = toAccount;this.amount = amount;}@Overridepublic void run() {while(true){synchronized (fromAccount){synchronized (toAccount){if(fromAccount.getBalance()>=amount) {fromAccount.debit(amount);toAccount.credit(amount);}System.out.println(Thread.currentThread().getName());System.out.println(fromAccount.getCountName()+"->"+toAccount.getCountName()+":"+amount );System.out.println(fromAccount.getCountName() +"账户余额:"+ fromAccount.getBalance());System.out.println(toAccount.getCountName() +"账户余额:"+ toAccount.getBalance());}}try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}}public static void main(String[] args) {Account bigHead = new Account("冤大头",100000);Account smallKill = new Account("门小抠",200000);new Thread(new TransferAccount(bigHead,smallKill,10)).start();new Thread(new TransferAccount(smallKill,bigHead,20)).start();}}

执行main方法后的输出结果：

Thread-0
冤大头->门小抠:10
冤大头账户余额:99990
门小抠账户余额:200010
Thread-1
门小抠->冤大头:20
门小抠账户余额:199990
冤大头账户余额:100010
Thread-1
门小抠->冤大头:20
门小抠账户余额:199970
冤大头账户余额:100030
Thread-0
冤大头->门小抠:10
冤大头账户余额:100020
门小抠账户余额:199980
Thread-0
冤大头->门小抠:10
冤大头账户余额:100010
门小抠账户余额:199990
Thread-1
门小抠->冤大头:20
门小抠账户余额:199970
冤大头账户余额:100030
Thread-0
冤大头->门小抠:10
冤大头账户余额:100020
门小抠账户余额:199980
Thread-1
门小抠->冤大头:20
门小抠账户余额:199960
冤大头账户余额:100040
Thread-0
冤大头->门小抠:10
冤大头账户余额:100030
门小抠账户余额:199970
Thread-1
门小抠->冤大头:20
门小抠账户余额:199950
冤大头账户余额:100050
Thread-0
冤大头->门小抠:10
冤大头账户余额:100040
门小抠账户余额:199960
Thread-1
门小抠->冤大头:20
门小抠账户余额:199940
冤大头账户余额:100060

这时，我们发现进程还在运行，但是控台停止输出了，它停在那里了。
截图如下：

这时我们使用 jps 来查看一下java进程：

D:\open_source\MyBatis\MyThread\target\classes\demo>jps
23600
24676 Launcher
40244 Jps
23672 TransferAccount

然后输入 使用jstack来看详情：

D:\open_source\MyBatis\MyThread\target\classes\demo>jstack 23672
2023-02-26 18:37:57
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.351-b10 mixed mode):"DestroyJavaVM" #14 prio=5 os_prio=0 tid=0x000001b4fc785800 nid=0x6530 waiting on condition [0x0000000000000000]java.lang.Thread.State: RUNNABLE"Thread-1" #13 prio=5 os_prio=0 tid=0x000001b4fc77f800 nid=0x887c waiting for monitor entry [0x000000467adff000]java.lang.Thread.State: BLOCKED (on object monitor)at deadlock.TransferAccount.run(TransferAccount.java:20)- waiting to lock <0x000000076c5a4610> (a deadlock.Account)- locked <0x000000076c5a4658> (a deadlock.Account)at java.lang.Thread.run(Thread.java:750)"Thread-0" #12 prio=5 os_prio=0 tid=0x000001b4fc77c800 nid=0x8c80 waiting for monitor entry [0x000000467acff000]java.lang.Thread.State: BLOCKED (on object monitor)at deadlock.TransferAccount.run(TransferAccount.java:20)- waiting to lock <0x000000076c5a4658> (a deadlock.Account)- locked <0x000000076c5a4610> (a deadlock.Account)at java.lang.Thread.run(Thread.java:750)......Found one Java-level deadlock:
=============================
"Thread-1":waiting to lock monitor 0x000001b4fa208eb8 (object 0x000000076c5a4610, a deadlock.Account),which is held by "Thread-0"
"Thread-0":waiting to lock monitor 0x000001b4fa208e08 (object 0x000000076c5a4658, a deadlock.Account),which is held by "Thread-1"Java stack information for the threads listed above:
===================================================
"Thread-1":at deadlock.TransferAccount.run(TransferAccount.java:20)- waiting to lock <0x000000076c5a4610> (a deadlock.Account)- locked <0x000000076c5a4658> (a deadlock.Account)at java.lang.Thread.run(Thread.java:750)
"Thread-0":at deadlock.TransferAccount.run(TransferAccount.java:20)- waiting to lock <0x000000076c5a4658> (a deadlock.Account)- locked <0x000000076c5a4610> (a deadlock.Account)at java.lang.Thread.run(Thread.java:750)Found 1 deadlock.

我们可以看到，其中Thread-0和Thread-1都已经处于BLOCK状态，发生了死锁。

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导致死锁发生，必须同时满足四个条件：互斥、占有且等待、不可抢占、循环等待。

• 互斥：共享资源A和B只能被一个线程占用。
• 占有且等待：线程Thread-1 已经取得共享资源X，在等待共享资源Y的时候，不释放共享资源X。
• 不可抢占：其他线程不能强行抢占线程Thread-1 占有的资源。
• 循环等待：线程Thread-1 等待线程Thread-2 占有的资源，线程Thread-2等待Thread-1 占有的资源。

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如何解决？

如果要解决死锁问题，只需要破坏其中一个条件，使其不满足即可。
除了互斥（多线程的基础）之外，其他三个条件都可以考虑破坏。
实际中遇到死锁，只能重启，如果还是死锁，要定位问题点，然后修复代码（破坏掉死锁必须满足的条件之一）后重新发布。

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我们来尝试破坏占有且等待的方式来破坏死锁：
新增加一个分配的类：Allocator，它有一个账户池。每个转账线程中要判断，账户池中是否已有此账户，如果没有可以转账，有的话就不转账。就可以避免共享资源同时存在于两个线程。

import java.util.ArrayList;
import java.util.List;public class Allocator {private List<Object> list = new ArrayList<>();synchronized boolean apply(Object fromAccount,Object toAccount){if(list.contains(fromAccount)||list.contains(toAccount)){return false;}list.add(fromAccount);list.add(toAccount);return true;}synchronized void free(Object fromAccount,Object toAccount){list.remove(fromAccount);list.remove(toAccount);}
}//相应的，TransAcount也要做修改：
public class TransferAccount implements Runnable {private Account fromAccount;private Account toAccount;private int amount;private Allocator allocator;public TransferAccount(Account fromAccount, Account toAccount, int amount, Allocator allocator) {this.fromAccount = fromAccount;this.toAccount = toAccount;this.amount = amount;this.allocator = allocator;}@Overridepublic void run() {while (true) {//判断账户是否已经分配过if (allocator.apply(fromAccount, toAccount)) {try {synchronized (fromAccount) {synchronized (toAccount) {if (fromAccount.getBalance() >= amount) {fromAccount.debit(amount);toAccount.credit(amount);}System.out.println(Thread.currentThread().getName());System.out.println(fromAccount.getCountName() + "->" + toAccount.getCountName() + ":" + amount);System.out.println(fromAccount.getCountName() + "账户余额:" + fromAccount.getBalance());System.out.println(toAccount.getCountName() + "账户余额:" + toAccount.getBalance());}}try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}finally {allocator.free(fromAccount,toAccount);}}}}public static void main(String[] args) {Account bigHead = new Account("冤大头", 100000);Account smallKill = new Account("门小抠", 200000);Allocator allocator = new Allocator();new Thread(new TransferAccount(bigHead, smallKill, 10, allocator)).start();new Thread(new TransferAccount(smallKill, bigHead, 20, allocator)).start();}}

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我们来避免第三个条件：使用Lock来替换掉 Synchronized。
Synchronized加锁后，要等到资源释放，而且锁是不可抢占的。
Lock中有个方法 tryLock，可以返回布尔值，如果返回fasle就不会进去。tryLock不会持续持有锁。

public class TransferAccount2 implements Runnable{private Account fromAccount;private Account toAccount;private int amount;private Lock fromAccountLock = new ReentrantLock();private Lock toAccountLock = new ReentrantLock();public TransferAccount2(Account fromAccount, Account toAccount, int amount) {this.fromAccount = fromAccount;this.toAccount = toAccount;this.amount = amount;}@Overridepublic void run() {while(true){//synchronized (fromAccount){if(fromAccountLock.tryLock()){if(toAccountLock.tryLock()){if(fromAccount.getBalance()>=amount) {fromAccount.debit(amount);toAccount.credit(amount);}System.out.println(Thread.currentThread().getName());System.out.println(fromAccount.getCountName()+"->"+toAccount.getCountName()+":"+amount );System.out.println(fromAccount.getCountName() +"账户余额:"+ fromAccount.getBalance());System.out.println(toAccount.getCountName() +"账户余额:"+ toAccount.getBalance());}}try {Thread.sleep(1000);} catch (InterruptedException e) {e.printStackTrace();}}}public static void main(String[] args) {Account bigHead = new Account("冤大头",100000);Account smallKill = new Account("门小抠",200000);new Thread(new TransferAccount2(bigHead,smallKill,10)).start();new Thread(new TransferAccount2(smallKill,bigHead,20)).start();}}

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