Java併發學習之九——使用本地線程變量

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1、如果創建一個類對象，實現Runnable接口，然後多個Thread對象使用同樣的Runnable對象，全部的線程都共享同樣的屬性。這意味着，如果你在一個線程裏改變一個屬性，全部的線程都會受到這個改變的影響。如果希望程序裏的哥哥線程的屬性不會被共享，Java併發API提供了一個很清楚的機制叫本地線程變量。

2、Java併發API包括Inheritable ThreadLocal類提供線程創建線程的值的遺傳性。如果線程A有一個本地線程變量，然後它創建了另一個線程B，那麼線程B將有與A相同的本地線程變量值。你可以覆蓋childValue方法來初始子線程的本地線程變量的值。它接收付線程的本地線程變量作爲參數。

下面先介紹一個不使用本地線程變量的代碼

package chapter;

import java.util.Date;
import java.util.concurrent.TimeUnit;

public class UnsafeTask implements Runnable{
	private Date startDate;
	@Override
	public void run() {
		startDate = new Date();
		System.out.printf("Starting Thread :%s:%s\n", Thread.currentThread().getId(),startDate);
		
		try {
			TimeUnit.SECONDS.sleep((int)Math.rint(Math.random()*10));
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.printf("Thread Finished:%s:%s\n", Thread.currentThread().getId(),startDate);
	}

}

package chapter;

import java.util.concurrent.TimeUnit;

public class Main9 {

	/**
	 * <p>
	 * </p>
	 * @author zhangjunshuai
	 * @date 2014-8-22 下午4:30:20
	 * @param args
	 */
	public static void main(String[] args) {

		UnsafeTask task = new UnsafeTask();
		for(int i = 0; i<10;i++){
			Thread thread = new Thread(task);
			thread.start();
			
			try {
				TimeUnit.SECONDS.sleep(2);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
		
		/*SafeTask task = new SafeTask();
		for(int i = 0; i<10;i++){
			Thread thread = new Thread(task);
			thread.start();
			
			try {
				TimeUnit.SECONDS.sleep(2);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}*/
	}

}

使用本地線程變量

package chapter;

import java.util.Date;
import java.util.concurrent.TimeUnit;

public class SafeTask implements Runnable{

	private static ThreadLocal<Date> startDate = new ThreadLocal<Date>(){
		protected Date initialValue(){
			return new Date();
		}
	};
	@Override
	public void run() {

		System.out.printf("Starting Thread:%s:%s\n", Thread.currentThread().getId(),startDate.get()
				);
		 try {
			TimeUnit.SECONDS.sleep((int)Math.rint(Math.random()*10));
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		 
		 System.out.printf("Thread Finished:%s:%s\n", Thread.currentThread().getId(),startDate.get());
	}
}

package chapter;

import java.util.concurrent.TimeUnit;

public class Main9 {

	/**
	 * <p>
	 * </p>
	 * @author zhangjunshuai
	 * @date 2014-8-22 下午4:30:20
	 * @param args
	 */
	public static void main(String[] args) {

		/*UnsafeTask task = new UnsafeTask();
		for(int i = 0; i<10;i++){
			Thread thread = new Thread(task);
			thread.start();
			
			try {
				TimeUnit.SECONDS.sleep(2);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}*/
		
		SafeTask task = new SafeTask();
		for(int i = 0; i<10;i++){
			Thread thread = new Thread(task);
			thread.start();
			
			try {
				TimeUnit.SECONDS.sleep(2);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}

}