1--Wrong conventional writing
<code>public static int i=0;</code><code>public static void add(){</code><code> i=i+1;</code><code> action();</code><code>}</code><code>public static void action(){</code><code> System.out.println("==>"+Thread.currentThread().getName()+":"+i);</code><code>}</code><code>public static void main(String[] args) throws InterruptedException {</code><code> Thread t1 = new Thread(SysUserServiceImpl::add,"t1");</code><code> Thread t2= new Thread(SysUserServiceImpl::add,"t2");</code><code> t1.start();</code><code> t2.start();</code><code>}</code><code>运行结果==></code><code>==>t1:1</code><code>==>t2:2</code><code><br></code><code>==>t1:2</code><code>==>t2:1</code><code><br></code><code>==>t1:2</code><code>==>t2:2</code>
The results are inconsistent each time. In a multi-threaded environment, t1 performs a 1 operation on i in the shared memory, but does not refresh the value to the main memory. At this time, t2 happens to also perform a 1 operation on i or 0. Operation, so that the final result i is all 1. In the same way, t1 is 1 after processing, and t2 is 2 after processing. The results of multiple runs are inconsistent.
Improvement method 1 --Synchronization lock
<code>public class ThreadException {</code><code> public static volatile int i=0;</code><code> public static void add(){</code><code> synchronized (ThreadException.class){</code><code> i=i+1;</code><code> action();</code><code> }</code><code> }</code><code> public static void action(){</code><code> System.out.println("==>"+Thread.currentThread().getName()+":"+i);</code><code> }</code><code> public static void main(String[] args) throws InterruptedException {</code><code> Thread t1 = new Thread(ThreadException::add,"t1");</code><code> Thread t2= new Thread(ThreadException::add,"t2");</code><code> t1.start();</code><code> t2.start();</code><code><br></code><code> }</code><code>}</code>
Advantages: simple implementation
Disadvantages: large locking granularity, low performance, distributed environment, multiple JVM conditions, synchronized failure, synchronized is only a local lock, and the lock is only the current one For objects under jvm, in distributed scenarios, distributed locks should be used
Improvement method 2 AtomicInteger
public class ThreadException { private static AtomicInteger num = new AtomicInteger(0); public static void add(){ int i = num.getAndIncrement(); action(i); } public static void action(int i){ System.out.println("由"+i+"==>"+Thread.currentThread().getName()+":"+num); } public static void main(String[] args) throws InterruptedException { Thread t1 = new Thread(ThreadException::add,"t1"); Thread t2= new Thread(ThreadException::add,"t2"); t1.start(); t2.start(); }}
Improvement method 3 lock
<code>public class ThreadException {</code><code> public static volatile int i=0;</code><code> public static void action(){</code><code> System.out.println("==>"+Thread.currentThread().getName()+":"+i);</code><code> }</code><code><br></code><code> static Lock lock=new ReentrantLock();</code><code> public static void inc() {</code><code> lock.lock();</code><code> try {</code><code> Thread.sleep(1);</code><code> i=i+1;</code><code> action();</code><code> } catch (InterruptedException e) {</code><code> e.printStackTrace();</code><code> } finally {</code><code> lock.unlock();</code><code> }</code><code> }</code><code> public static void main(String[] args) throws InterruptedException {</code><code> Thread t1 = new Thread(ThreadException::inc,"t1");</code><code> Thread t2= new Thread(ThreadException::inc,"t2");</code><code> t1.start();</code><code> t2.start();</code><code> }</code><code>}</code><code><br></code>
Distributed lock: ensure the synchronous execution of multiple nodes
Implementation plan: 1. Based on database, 2. Based on redis cache, 3. Based on zookeeper
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