Java編程多線程之共享數(shù)據(jù)代碼詳解

來源：本站原創(chuàng)|時間：2020-01-10|欄目：Java編程|點擊：次

本文主要總結線程共享數(shù)據(jù)的相關知識，主要包括兩方面:一是某個線程內(nèi)如何共享數(shù)據(jù)，保證各個線程的數(shù)據(jù)不交叉；一是多個線程間如何共享數(shù)據(jù)，保證數(shù)據(jù)的一致性。

線程范圍內(nèi)共享數(shù)據(jù)

自己實現(xiàn)的話，是定義一個Map,線程為鍵，數(shù)據(jù)為值,表中的每一項即是為每個線程準備的數(shù)據(jù),這樣在一個線程中數(shù)據(jù)是一致的。

例子

package com.iot.thread;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
/**
 * Created by brian on 2016/2/4.
 */
public class ThreadScopeShareData {
	//準備一個哈希表，為每個線程準備數(shù)據(jù)
	private static Map<Thread,Integer> threadData = new HashMap<>();
	public static void main(String[] args) {
		for (int i=0;i<2;i++){
			new Thread(
			          new Runnable() {
				@Override
				        public void run() {
					int data = new Random().nextint();
					threadData.put(Thread.currentThread(),data);
					System.out.println(Thread.currentThread()+" put data："+data);
					new A().get();
					new B().get();
				}
			}
			).start();
		}
	}
	static class A{
		public void get(){
			int data = threadData.get(Thread.currentThread());
			System.out.println("A from "+Thread.currentThread()+" get data "+data);
		}
	}
	static class B{
		public void get(){
			int data = threadData.get(Thread.currentThread());
			System.out.println("B from "+Thread.currentThread()+" get data "+data);
		}
	}
}

上述代碼偶爾會報異常：

Exception in thread "Thread-0" java.lang.NullPointerException
at com.iot.thread.ThreadScopeShareData$A.get(ThreadScopeShareData.java:29)
at com.iot.thread.ThreadScopeShareData$1.run(ThreadScopeShareData.java:21)
at java.lang.Thread.run(Thread.java:745)

具體原因還不知道

ThreadLocal類

API:

java.lang:Class ThreadLocal<T>

單變量

使用ThreadLocal類型的對象代替上面的Map即可

多變量

定義一個對象來封裝多個變量，然后在ThreadLocal中存儲整個對象

多變量時，最好將ThreadLocal類放在數(shù)據(jù)類的內(nèi)部，數(shù)據(jù)類采用單例模式，這樣，新建對象和獲取對象都會更方便，同時封裝性更強。

示例代碼:

package com.iot.thread;
import java.util.Random;
/**
 * Created by brian on 2016/2/4.
 */
public class ThreadLocalTest {
	private static ThreadLocal<Integer> threadInger = new ThreadLocal<>();
	public static void main(String[] args) {
		for (int i=0;i<2;i++){
			new Thread(new Runnable() {
				@Override
				        public void run() {
					int data = new Random().nextint(100);
					threadInger.set(data);
					System.out.println(Thread.currentThread()+" put data："+data);
					MyThreadScopeData.getThreadInstance().setName(Thread.currentThread().toString());
					MyThreadScopeData.getThreadInstance().setAge(data%10);
					new A().get();
					new B().get();
				}
			}
			).start();
		}
	}
	static class A{
		public void get(){
			int data = threadInger.get();
			System.out.println("A from "+Thread.currentThread()+" get data "+data);
			MyThreadScopeData myThreadScopeData = MyThreadScopeData.getThreadInstance();
			System.out.println("A from "+myThreadScopeData);
		}
	}
	static class B{
		public void get(){
			int data = threadInger.get();
			System.out.println("B from "+Thread.currentThread()+" get data "+data);
			MyThreadScopeData myThreadScopeData = MyThreadScopeData.getThreadInstance();
			System.out.println("B from "+myThreadScopeData);
		}
	}
}
/**
 * 將多變量封裝起來的數(shù)據(jù)類
 * 單例模式，內(nèi)置ThreadLocal類型變量
 */
class MyThreadScopeData{
	private MyThreadScopeData(){
	}
	private static ThreadLocal<MyThreadScopeData> data = new ThreadLocal<>();
	public static MyThreadScopeData getThreadInstance(){
		MyThreadScopeData instance = data.get();
		if(instance == null){
			instance = new MyThreadScopeData();
			data.set(instance);
		}
		return instance;
	}
	private String name;
	private int age;
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	@Override
	  public String toString() {
		String reVal = super.toString()+"-{name,age}"+":{"+getName()+","+getAge()+"}";
		return reVal;
	}
}

多線程訪問共享數(shù)據(jù)

幾種方式

線程執(zhí)行代碼相同，使用同一Runnable對象，Runnable對象中有共享數(shù)據(jù)
線程執(zhí)行代碼不同，將共享數(shù)據(jù)封裝在另一對象中（操作數(shù)據(jù)的方法也在該對象完成），將這個對象逐一傳遞給各個Runnable對象。[本質(zhì)：共享數(shù)據(jù)的對象作為參數(shù)傳入Runnable對象]
線程執(zhí)行代碼不同，將Runnable對象作為某一個類的內(nèi)部類，共享數(shù)據(jù)作為這個外部類的成員變量（操作數(shù)據(jù)的方法放在外部類）。[本質(zhì):不同內(nèi)部類共享外部類數(shù)據(jù)]
結合上兩種方式，將共享數(shù)據(jù)封裝在另一對象中（操作數(shù)據(jù)的方法也在該對象完成），該對象作為這個外部類的成員變量，將Runnable對象作為內(nèi)部類

最后一種方式的示例:

設計5個線程，其中三個線程每次對j增加1，另外兩個線程對j每次減少1

package com.iot.thread;
/**
 * Created by brian on 2016/2/4.
 */
public class MutiThreadShareData {
	private static MutiShareData mutiShareData = new MutiShareData();
	public static void main(String[] args) {
		for (int i=0;i<3;i++){
			new Thread(
			          new Runnable() {
				@Override
				            public void run() {
					System.out.println(Thread.currentThread()+":{j from "+ mutiShareData.getJ()+" + to: "+mutiShareData.increment()+"}");
				}
			}
			).start();
		}
		for (int i=0;i<2;i++){
			new Thread(
			          new Runnable() {
				@Override
				            public void run() {
					System.out.println(Thread.currentThread()+":{j from "+ mutiShareData.getJ()+" - to: "+mutiShareData.decrement()+"}");
				}
			}
			).start();
		}
	}
}
/**
 * 將共享數(shù)據(jù)封裝在另一對象中（操作數(shù)據(jù)的方法也在該對象完成）
 */
class MutiShareData{
	private int j = 0;
	public synchronized int increment(){
		return ++j;
	}
	public synchronized int decrement(){
		return --j;
	}
	public synchronized int getJ() {
		return j;
	}
	public synchronized void setJ(int j) {
		this.j = j;
	}
}