synchronized | 易学教程

Synchronizing on local variable

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问题 I noticed a weird construct in ConcurrentHashMap's compute and computeIfAbsent methods: Node<K,V> r = new ReservationNode<K,V>(); synchronized (r) { //... } What is the point of synchronizing on a local object considering that the JIT will most likely treat it as a no-op? 回答1: Right after the code has acquired the object’s monitor, the reference to the object is stored into the tab which is the globally visible array of nodes which make up the contents of the ConcurrentHashMap : Node<K,V> r =

线程池，多线程，线程异步，同步和死锁，Lock接口

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线程池　　线程池，其实就是一个容纳多个线程的容器，其中的线程可以反复使用，省去了频繁创建线程对象的操作，无需反复创建线程而消耗过多资源。除了创建和销毁线程的开销之外，活动的线程也需要消耗系统资源。线程池主要用来解决线程生命周期开销问题和资源不足问题。使用线程池方式--Runnable接口通常，线程池都是通过线程池工厂创建，再调用线程池中的方法获取线程，再通过线程去执行任务方法。 Executors：线程池创建工厂类 public static ExecutorService newFixedThreadPool(int nThreads)：返回线程池对象 ExecutorService：线程池类 Future<?> submit(Runnable task)：获取线程池中的某一个线程对象，并执行 Future接口：用来记录线程任务执行完毕后产生的结果。线程池创建与使用使用线程池中线程对象的步骤： 1. 创建线程池对象 2. 创建Runnable接口子类对象 3. 提交Runnable接口子类对象 4. 关闭线程池 package com.oracle.xiancheng; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class

Difference between synchronization of field reads and volatile

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问题 In a nice article with some concurrency tips, an example was optimized to the following lines: double getBalance() { Account acct = verify(name, password); synchronized(acct) { return acct.balance; } } If I understand that correctly, the point of the synchronization is to ensure that the value of acct.balance that are read by this thread is current and that any pending writes to the fields of the object in acct.balance are also written to main memory. The example made me think a little:

static Webdriver instance synchronization in java

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问题 GlobalVariables class holds different variables which are used across my framework one of them is WebDriver instance: public class GlobalVariables { public static WebDriver driver; //Some other static global variables required across my framework public GlobalVariables(String propertiesFile) { initializeVariables(propertiesFile); } public void initializeVariables(String propertiesFile) { GlobalInitializer obj=new GlobalInitializer(); obj.initialize(String propertiesFile); } }

Learning Java, use of synchronized keyword

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问题 so i was testing with synchronized keyword. Here is an example that I tried: public class MyTest { static int i = 0; public static void main(String[] args) { new Thread(t1).start(); new Thread(t2).start(); } private static void countMe(String name){ i++; System.out.println("Current Counter is: " + i + ", updated by: " + name); } private static Runnable t1 = new Runnable() { public void run() { try{ for(int i=0; i<5; i++){ countMe("t1"); } } catch (Exception e){} } }; private static Runnable

Learning Java, use of synchronized keyword

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Why are synchronize expensive in Java?

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问题 I am really new to Java and I read that synchronized is "very expensive" in Java. All I want to know is what is expensive and how is it expensive? Thanks. 回答1: Maybe it's not as bad as you think It used to be terrible (which is possibly why you read that it was "very expensive"). These memes can take a long time to die out How expensive is synchronization? Because of the rules involving cache flushing and invalidation, a synchronized block in the Java language is generally more expensive than

Java线程机制

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线程简介：线程是一个程序内部的顺序控制流。线程和进程的区别：　　每个进程都有独立的代码和数据空间（进程上下文），进程间的切换会有较大的开销。　　线程可以看成是轻量级的进程，同一类线程共享代码和数据空间，每个线程有独立的运行栈和程序计数器，线程切换的开销小。多进程：　　在操作系统中能同时运行的多个任务。多线程：　　在同一个应用程序中有多个顺序流同时执行。 Java的线程是通过java.lang.Thread类实现的；JVM启动时会有一个由主方法所定义的线程；可以通过创建Thread的实例来创建新的线程；每个线程都是通过某个特定的Thread对象所对应的run()方法来完成其操作的，run()方法称为线程体。线程的创建与启动：第一种：　　1.定义线程类target实现Runnable接口（使用Runnable接口可以为多个线程提供共享数据），其中Runnable中只有一个方法public void run(); 　　2.Thread my Thread = new Thread(target); 　　3.在实现Runnable接口的类的run()方法定义中可以使用Thread的静态方法（currectThread()方法用于获取当前线程的引用）。第二种：　　1.可以定义一个Thread的子类并重写其run()方法：class MyThread extends

Should you synchronize the run method? Why or why not?

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问题 I have always thought that synchronizing the run method in a java class which implements Runnable is redundant. I am trying to figure out why people do this: public class ThreadedClass implements Runnable{ //other stuff public synchronized void run(){ while(true) //do some stuff in a thread } } } It seems redundant and unnecessary since they are obtaining the object's lock for another thread. Or rather, they are making explicit that only one thread has access to the run() method. But since

What is the difference between synchronized on lockObject and using this as the lock?

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问题 I know the difference between synchronized method and synchronized block but I am not sure about the synchronized block part. Assuming I have this code class Test { private int x=0; private Object lockObject = new Object(); public void incBlock() { synchronized(lockObject) { x++; } System.out.println("x="+x); } public void incThis() { // same as synchronized method synchronized(this) { x++; } System.out.println("x="+x); } } In this case what is the difference between using lockObject and