Examples of using thread locks and read-write locks in Java multi-threaded programming

Thread lock Lock
Lock is equivalent to the Synchronized

import java.util.concurrent.locks.Lock; 
import java.util.concurrent.locks.ReentrantLock; 
/* 
 * Lock lock = new ReentrantLock(); 
 * lock.lock();  lock.unLock(); 
 * 类似于 synchronized，但不能与synchronized 混用 
 */
public class LockTest { 
  public static void main(String[] args) { 
    final Outer outer = new LockTest().new Outer(); 
    new Thread(new Runnable() { 
  
      @Override
      public void run() { 
        // TODO Auto-generated method stub 
        while (true) { 
          outer.out1("zhangxiaoxiao"); 
          outer.out2("lihuoming"); 
        } 
      } 
  
    }).start(); 
    new Thread(new Runnable() { 
  
      @Override
      public void run() { 
        // TODO Auto-generated method stub 
        while (true) { 
          outer.out1("zhangxiaoxiao"); 
          outer.out2("lihuoming"); 
        } 
      } 
  
    }).start(); 
  
  } 
  class Outer { 
    Lock lock = new ReentrantLock(); 
    void out1(String name) { 
      lock.lock(); 
      int len = name.length(); 
      for (int i = 0; i < len; i++) { 
        System.out.print(name.charAt(i)); 
      } 
      System.out.println(); 
      lock.unlock(); 
    } 
    void out2(String name) { 
      lock.lock(); 
      int len = name.length(); 
      for (int i = 0; i < len; i++) { 
        System.out.print(name.charAt(i)); 
      } 
      System.out.println(); 
      lock.unlock(); 
    } 
  } 
}

read-write lock of the current object ReentrantReadWriteLock

Read-write lock: allows multiple threads to read at the same time; cannot read while writing; cannot write while reading; cannot write while writing

import java.util.HashMap; 
import java.util.Map; 
import java.util.concurrent.locks.ReadWriteLock; 
import java.util.concurrent.locks.ReentrantReadWriteLock; 
/* 
 * 问题：设计一个 缓存机制 的 伪代码 
 *  从集合中读取数据，读取不到，就写入，再考虑多线程并发的情况  
 */ 
public class ReadWriteLockTest2 { 
  private Map<String, Object> data = new HashMap<String, Object>(); 
  private ReadWriteLock lock = new ReentrantReadWriteLock();  
  //读写锁 
  public Object getInstance1(String key, Object obj) { 
    lock.readLock().lock(); 
    Object value = null; 
    try { 
      value = data.get(key); 
      if (value == null) {//一条线程进入这里， 
        lock.readLock().unlock();//读锁解了，其他线程就可能也到这了 
        lock.writeLock().lock();//只能有一条线程到这，写锁开启，要写入数据了 
        if (data.get(key) == null) { 
          value = obj;   
            /*先判断数据，如果不存在，才写入，，因为 界时当写锁解除后，其他 
              线程，再想写入时， 判断到数据存在 ，就跳过了。就不会发生同时写入的情况 
            */   
          data.put(key, value);  
        } 
        lock.writeLock().unlock();//写完了，解锁 
        lock.readLock().lock(); 
      } 
    } finally { 
      lock.readLock().unlock(); 
    }   
    return value; 
  }

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