Java線程是一項非常基本和重要的技術,在偏底層和偏技術的Java程序中不可避免地要使用到Java多線程技術,那麼數據的共享也就是我們必須考慮的問題之一,自然我也就會想到ThreadLocal和synchronized。
ThreadLocal使用場合主要解決多線程中數據因併發產生不一致問題。ThreadLocal爲每個線程中併發訪問的數據提供一個副本,通過訪問副本來運行業務,這樣的結果雖然耗費一些內存,但可以大大降低線程同步所帶來性能消耗,同時也減少了線程併發控制的複雜度。synchronized是利用鎖的機制,使變量或代碼塊在某一時該只能被一個線程訪問。二者雖然都是解決多線程併發訪問的問題,但是區別很大。Synchronized用於線程間的數據共享,而ThreadLocal則用於線程間的數據隔離。下面是一個小例子,使用ThreadLocal時我們可以參照一下它的寫法:
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
public class ThreadLocalTest {
private static ThreadLocal<Integer> x = new ThreadLocal<Integer>();
private static ThreadLocal<MyThreadScopeData> myThreadScopeData = new ThreadLocal<MyThreadScopeData>();
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();
System.out.println(Thread.currentThread().getName()
+ " has put data :" + data);
x.set(data);
MyThreadScopeData.getThreadInstance().setName("name" + data);
MyThreadScopeData.getThreadInstance().setAge(data);
new A().get();
new B().get();
}
}).start();
}
}
static class A{
public void get(){
int data = x.get();
System.out.println("A from " + Thread.currentThread().getName()
+ " get data :" + data);
MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
System.out.println("A from " + Thread.currentThread().getName()
+ " getMyData: " + myData.getName() + "," +
myData.getAge());
}
}
static class B{
public void get(){
int data = x.get();
System.out.println("B from " + Thread.currentThread().getName()
+ " get data :" + data);
MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
System.out.println("B from " + Thread.currentThread().getName()
+ " getMyData: " + myData.getName() + "," +
myData.getAge());
}
}
}
class MyThreadScopeData{
private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
private String name;
private int age;
private MyThreadScopeData(){}
public static /*synchronized*/ MyThreadScopeData getThreadInstance(){
MyThreadScopeData instance = map.get();
if(instance == null){
instance = new MyThreadScopeData();
map.set(instance);
}
return instance;
}
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;
}
}
總結:synchronized是利用鎖的機制使變量或代碼塊在某一時該只能被一個線程訪問。而ThreadLocal卻爲每一個線程都提供了變量的副本,使得每個線程在某一時間訪問到的並不是同一個對象,這樣就隔離了多個線程對數據的數據共享。而Synchronized正好相反,它用於在多個線程間通信時能夠獲得數據共享。具體使用什麼我們要看編程時的應用環境而定。