Principle

Use the higher-level concurrency utilities instead of wait and notify for easiness.

Use ConcurrentHashMap in preference to Collections.synchronizedMap or Hashtable.

Use concurrent collections in preference to externally synchronized collections.

Three categories of higher-level utilities in java.util.concurrent

  1. Executor Framework (Item 68)
  2. Concurrent collections - provide high- performance concurrent implementations of standard collection interfaces such as List, Queue, and Map.   

    Since all the implementation of Concurrent collections manage their own synchronization internally it's impossible to exclude concurrent activity from a concurrent collection; locking it will have no effect but slow the program.

// Method simulates the behavior of String.intern. Concurrent canonicalizing map atop ConcurrentMap - faster!

private static final ConcurrentMap<String, String> map = new ConcurrentHashMap<String, String>();

public static String intern(String s) {

String result = map.get(s);

if (result == null) {

result = map.putIfAbsent(s, s);

if (result == null)

result = s;

}

return result;

}

Note

String.intern must use some sort of weak reference to keep from leaking memory over time.

Blocking operation - wait until they can be successfully performed.

BlockingQueue (Used for work queues) extends Queue and adds several methods, including take, which removes and returns the head element from the queue, waiting if the queue is empty.

  1. Synchronizers - Objects that enable threads to wait for one another.(eg. CountDownLatch, Semaphore, CyclicBarrier and Exchanger).   

    Countdown latches are single-use barriers that allow one or more threads to wait for one or more other threads to do something.

    /**

    * Concurrency timer demo for "69 Prefer concurrency utilities to wait and notify".

    */

    package com.effectivejava.concurrency;

    import java.util.concurrent.CountDownLatch;

    import java.util.concurrent.Executor;

    import java.util.concurrent.SynchronousQueue;

    import java.util.concurrent.ThreadPoolExecutor;

    import java.util.concurrent.TimeUnit;

    /**

    * @author Kaibo Hao

    *

    */

    public class ExecutorManager {

    // Simple framework for timing concurrent execution

    public static long time(Executor executor, int concurrency,

    final Runnable action) throws InterruptedException {

    final CountDownLatch ready = new CountDownLatch(concurrency);

    final CountDownLatch start = new CountDownLatch(1);

    final CountDownLatch done = new CountDownLatch(concurrency);

    for (int i = 0; i < concurrency; i++) {

    executor.execute(new Runnable() {

    public void run() {

    ready.countDown(); // Tell timer we're ready

    try {

    start.await(); // Wait till peers are ready

    action.run();

    } catch (InterruptedException e) {

    Thread.currentThread().interrupt();

    } finally {

    done.countDown(); // Tell timer we're done

    }

    }

    });

    }

    ready.await(); // Wait for all workers to be ready

    long startNanos = System.nanoTime();

    start.countDown(); // And they're off!

    done.await(); // Wait for all workers to finish

    return System.nanoTime() - startNanos;

    }

    /**

    * @param args

    */

    public static void main(String[] args) {

    try {

    Executor executor = new ThreadPoolExecutor(0, 2, 10,

    TimeUnit.MILLISECONDS, new SynchronousQueue<Runnable>());

    long executedTime = time(executor, 2, new Runnable() {

    @Override

    public void run() {

    System.out.printf("Runing %s%n", Thread.currentThread());

    }

    });

    System.out.printf("%sns %.3fms %.3fs", executedTime,

    executedTime / 1000.0, executedTime / 1000000.0);

    } catch (InterruptedException e) {

    e.printStackTrace();

    }

    }

    }

Note

If a worker thread catches an InterruptedException, it reasserts the interrupt using the idiom Thread.currentThread().interrupt() and returns from its run method.

Since System.nanoTime is both more accurate and more precise, and it is not affected by adjustments to the system's real-time clock. For interval timing, always use System.nanoTime in preference to System.currentTimeMillis.

Always use the wait loop idiom to invoke the wait method; never invoke it outside of a loop.

// The standard idiom for using the wait method

synchronized (obj) {

while (<condition does not hold>)

obj.wait(); // (Releases lock, and reacquires on wakeup)

... // Perform action appropriate to condition

}

Reasons a thread might wake up when the condition does not hold:

• Another thread could have obtained the lock and changed the guarded state between the time a thread invoked notify and the time the waiting thread woke.

• Another thread could have invoked notify accidentally or maliciously when the condition did not hold. Classes expose themselves to this sort of mischief by waiting on publicly accessible objects. Any wait contained in a synchronized method of a publicly accessible object is susceptible to this problem.

• The notifying thread could be overly "generous" in waking waiting threads. For example, the notifying thread might invoke notifyAll even if only some of the waiting threads have their condition satisfied.

• The waiting thread could (rarely) wake up in the absence of a notify. This is known as a spurious wakeup[Posix, 11.4.3.6.1; JavaSE6].

Summary

using wait and notify directly is like programming in "concurrency assembly language," as compared to the higher-level language provided by java.util.concurrent. There is seldom, if ever, a reason to use wait and notify in new code. If you maintain code that uses wait and notify, make sure that it always invokes wait from within a while loop using the standard idiom. The notifyAll method should generally be used in preference to notify. If notify is used, great care must be taken to ensure liveness.

Effective Java 69 Prefer concurrency utilities to wait and notify的更多相关文章

  1. Effective Java 35 Prefer annotations to naming patterns

    Disadvantages of naming patterns Typographical errors may result in silent failures. There is no way ...

  2. Effective Java 53 Prefer interfaces to reflection

    Disadvantage of reflection You lose all the benefits of compile-time type checking, including except ...

  3. Effective Java 68 Prefer executors and tasks to threads

    Principle The general mechanism for executing tasks is the executor service. If you think in terms o ...

  4. Effective Java 18 Prefer interfaces to abstract classes

    Feature Interface Abstract class Defining a type that permits multiple implementations Y Y Permitted ...

  5. Effective Java 20 Prefer class hierarchies to tagged classes

    Disadvantage of tagged classes 1. Verbose (each instance has unnecessary irrelevant fields). 2. Erro ...

  6. Effective Java 25 Prefer lists to arrays

    Difference Arrays Lists 1 Covariant Invariant 2 Reified at runtime Erased at run time 3 Runtime type ...

  7. Effective Java 46 Prefer for-each loops to traditional for loops

    Prior to release 1.5, this was the preferred idiom for iterating over a collection: // No longer the ...

  8. Effective Java 49 Prefer primitive types to boxed primitives

    No. Primitives Boxed Primitives 1 Have their own values Have identities distinct from their values 2 ...

  9. Effective Java Index

    Hi guys, I am happy to tell you that I am moving to the open source world. And Java is the 1st langu ...

随机推荐

  1. [Python] raw_input

    该函数输入的是字符串,如果想输入数字,可以用强制转换.

  2. 设计模式之Iterator模式(2)

    这篇文章比较简单,作一个笔记. 模拟Iterator. Iterator接口: package cn.asto.Interator; public interface Iterator { publi ...

  3. GitLab安装说明

    GitLab,是一个使用 Ruby on Rails 开发的开源应用程序,与Github类似,能够浏览源代码,管理缺陷和注释,非常适合在团队内部使用. gitlab是基于Ruby on Rails的, ...

  4. mysql数据库入门

    在很多地方都有人提到MySQL这个数据,之前没有接触过的mysql数据库的童鞋们可以跟我一起走进mysql的世界. http://hovertree.com/menu/mysql/ 安装我就不多说了, ...

  5. java数据库查询类

    通用查询数据库辅助类,可实现任意查询语句的查询,还可以进行多结果集查询. 类的代码: package com.hongyuan.db; import java.math.BigDecimal; imp ...

  6. 与众不同 windows phone 8.0 & 8.1 系列文章索引

    [源码下载] [与众不同 windows phone 7.5 (sdk 7.1) 系列文章索引] 与众不同 windows phone 8.0 & 8.1 系列文章索引 作者:webabcd ...

  7. SqlServer根据时段统计数据

    create table ST_PPTN_R_Real ( TID int primary key identity(1,1), STCD varchar(100) not null, TM date ...

  8. jquery 全选 全不选 反选

    1.概述 在项目中经常遇到列表中对复选框进行勾选操作,全选...反选.. 2. example <html> <body> <form id="test-for ...

  9. mysql max_allowed_packet过小导致的prepare失败

    最近公司一台阿里云上模拟环境突然好好地就出错了额,总提示:"Unknown prepared statement handler (stmt) given to DEALLOCATE PRE ...

  10. H5前端面试题及答案(1)

    前几天去面试了一家公司,整下改公司的面试题. 1.新的 HTML5 文档类型和字符集是? HTML5 文档类型很简单: <!doctype html> HTML5 使用 UTF-8 编码示 ...