为什么要引入线程

在过去当我们访问一个ftp站点时，经常会出现打不开的现象。是因为机器不够好、网速不够快么？不是，最大的原因在于，我们每一个用户访问服务器，服务器都会把我们当作一个进程，而每个进程是要分配资源的，也就是说当有100个用户同时访问站点，服务器就有100个进程需要处理。而进程是一种heavyweight item，所以比较容易将服务器搞垮。

Whereas each separate process has its own block of memeory, threads are easier on resources because they share memory.

another factor of three in server performance

Your server can run nine times faster.
Fewer than a hundred threads can handle thousands of short connections per minutes.
The impact of running many different threads on the server hardware is relatively minimal.

cost

developers should concern issues of safety and liveness.Because different threads share the same memory.
Different threads have to be extremely careful about which resources they use when.
Attention to deadlock.

Running Threads

api

Start a thread

Thread t = new Thread();
t.start();

But it has nothing to do.

Two ways to give a thread something to do

subclass the Thread class and override its public void run() method.

/*For example, a thread that computes primes larger than a stated value could be written as follows:*/
class PrimeThread extends Thread {
     long minPrime;
     PrimeThread(long minPrime) {
         this.minPrime = minPrime;
     }

     public void run() {
         // compute primes larger than minPrime
          . . .
     }
 }
 /*The following code would then create a thread and start it running:*/
 PrimeThread p = new PrimeThread(143);
 p.start();

Implement the Runnable interface and pass the Runnable object to the Thread constructor.(preferred)

/*The same example in this other style looks like the following:*/
class PrimeRun implements Runnable {
     long minPrime;
     PrimeRun(long minPrime) {
         this.minPrime = minPrime;
     }

     public void run() {
         // compute primes larger than minPrime
          . . .
     }
 }
 /*The following code would then create a thread and start it running:*/
 PrimeRun p = new PrimeRun(143);
 new Thread(p).start();

subclass

We are going to implement a subclass of Thread whose run() method calculates a 256-bit SHA-2 message digest for a specified file.

线程(1)

线程