threadlocal和static的区别

ThreadLocal类很简单，只有4个方法

void set(Object value);

设置当前线程的线程局部变量的值

public Object get()

该方法返回当前线程所对应的线程局部变量

public void remove()

protected Object initialValue()

这个方法是一个延迟调用方法，在线程第一次调用get()或set(Object)时才执行，并且执行1次。ThreadLocal中默认实现直接返回一个Null

ThreadLocal是为解决多线程程序的并发问题而提出的，可以称之为线程局部变量。与一般的变量的区别在于，生命周期是在线程范围内的。
static变量是的生命周期与类的使用周期相同，即只要类存在，那么static变量也就存在。
那么一个 static 的 ThreadLocal会是什么样的呢？

看下面一个例子，

public class SequenceNumber {

 private static ThreadLocal<Integer> seqNum = new ThreadLocal<Integer>(){
  public Integer initialValue(){
   return 0;
  }
 };

 public int getNextNum(){
  seqNum.set(seqNum.get() + 1);  //执行iniialValue
  return seqNum.get();
 }

 public static void main(String[] args){
  SequenceNumber sn = new SequenceNumber();
  TestClient t1  = new TestClient(sn);
  TestClient t2  = new TestClient(sn);
  TestClient t3  = new TestClient(sn);

  t1.start();
  t2.start();
  t3.start();

  t1.print();
  t2.print();
  t3.print();


 }

 private static class TestClient extends Thread{
  private SequenceNumber sn ;
  public TestClient(SequenceNumber sn ){
   this.sn = sn;
  }

  public void run(){
   for(int i=0; i< 3; i++){
    System.out.println( Thread.currentThread().getName()  + " --> " + sn.getNextNum());
   }
  }

  public void print(){
   for(int i=0; i< 3; i++){
    System.out.println( Thread.currentThread().getName()  + " --> " + sn.getNextNum());
   }
  }
 }

}

public class SequenceNumber { private static ThreadLocal<Integer> seqNum = new ThreadLocal<Integer>(){ public Integer initialValue(){ return 0; } }; public int getNextNum(){ seqNum.set(seqNum.get() + 1); return seqNum.get(); } public static void main(String[] args){ SequenceNumber sn = new SequenceNumber(); TestClient t1 = new TestClient(sn); TestClient t2 = new TestClient(sn); TestClient t3 = new TestClient(sn); t1.start(); t2.start(); t3.start(); t1.print(); t2.print(); t3.print(); } private static class TestClient extends Thread{ private SequenceNumber sn ; public TestClient(SequenceNumber sn ){ this.sn = sn; } public void run(){ for(int i=0; i< 3; i++){ System.out.println( Thread.currentThread().getName() + " --> " + sn.getNextNum()); } } public void print(){ for(int i=0; i< 3; i++){ System.out.println( Thread.currentThread().getName() + " --> " + sn.getNextNum()); } } } }

下面是结果

Thread-2 --> 1
Thread-2 --> 2
Thread-2 --> 3
Thread-0 --> 1
Thread-0 --> 2
Thread-0 --> 3
Thread-1 --> 1
Thread-1 --> 2
Thread-1 --> 3
main --> 1
main --> 2
main --> 3
main --> 4
main --> 5
main --> 6
main --> 7
main --> 8
main --> 9

Thread-2 --> 1 Thread-2 --> 2 Thread-2 --> 3 Thread-0 --> 1 Thread-0 --> 2 Thread-0 --> 3 Thread-1 --> 1 Thread-1 --> 2 Thread-1 --> 3 main --> 1 main --> 2 main --> 3 main --> 4 main --> 5 main --> 6 main --> 7 main --> 8 main --> 9

可以发现，static的ThreadLocal变量是一个与线程相关的静态变量，即一个线程内，static变量是被各个实例共同引用的，但是不同线程内，static变量是隔开的

public class JackyDao

{

private static ThreadLocal<Connection> connThreadLocal=new ThreadLocal<Connection>();

pubic static Connection getConnection()

{

if(connThreadLocal.get()==null)

{

Connection conn=ConnectionManager.getConnection();

connThreadLocal.set(conn);

return conn;

}

else

{

return connThreadLocal.get();

}

}

}