什么是线程安全线程？
在Java里，线程安全一般体现在两个方面：
1、多个thread对同一个java实例的访问（read和modify）不会相互干扰，它主要体现在关键字synchronized。如ArrayList和Vector，HashMap和Hashtable（后者每个方法前都有synchronized关键字）。如果你在interator一个List对象时，其它线程remove一个element，问题就出现了。
2、每个线程都有自己的字段，而不会在多个线程之间共享。它主要体现在java.lang.ThreadLocal类，而没有Java关键字支持，如像static、transient那样。
一个普遍的疑问，我们的Servlet中能够像JavaBean那样declare instance或static字段吗？如果不可以？会引发什么问题？
答案是：不可以。我们下面以实例讲解：
首先，我们写一个普通的Servlet，里面有instance字段count：
public class SimpleServlet extends HttpServlet
{
// A variable that is NOT thread-safe!
private int counter = 0;
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
{
doPost(req, resp);
}
public void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException
{
resp.getWriter().println("<HTML><BODY>");
resp.getWriter().println(this + " ==> ");
resp.getWriter().println(Thread.currentThread() + ": <br>");
for (int c = 0; c < 10; c++)
{
resp.getWriter().println("Counter = " + counter + "<BR>");
try
{
Thread.sleep((long) Math.random() * 1000);
counter++;
}
catch (InterruptedException exc)
{
}
}
resp.getWriter().println("</BODY></HTML>");
}
}<HTML>
<BODY>
<TABLE>
<TR>
<TD><IFRAME src="./SimpleServlet" name="servlet1" height="200%"> </IFRAME></TD>
</TR>
<TR>
<TD><IFRAME src="./SimpleServlet" name="servlet2" height="200%"> </IFRAME></TD>
</TR>
<TR>
<TD><IFRAME src="./SimpleServlet" name="servlet3" height="200%"> </IFRAME></TD>
</TR>
</TABLE>
</BODY>
</HTML>刷新页面几次后，产生的结果为：
com.zwchen.servlet.SimpleServlet@11e1bbf ==> Thread[http-8081-Processor23,5,main]:
Counter = 60
Counter = 61
Counter = 62
Counter = 65
Counter = 68
Counter = 71
Counter = 74
Counter = 77
Counter = 80
Counter = 83
com.zwchen.servlet.SimpleServlet@11e1bbf ==> Thread[http-8081-Processor22,5,main]:
Counter = 61
Counter = 63
Counter = 66
Counter = 69
Counter = 72
Counter = 75
Counter = 78
Counter = 81
Counter = 84
Counter = 87
com.zwchen.servlet.SimpleServlet@11e1bbf ==> Thread[http-8081-Processor24,5,main]:
Counter = 61
Counter = 64
Counter = 67
Counter = 70
Counter = 73
Counter = 76
Counter = 79
Counter = 82
Counter = 85
Counter = 88
我们会发现三点：
servlet只产生了一个Servlet对象，因为输出this时，其hashcode都一样，
servlet在不同的线程（线程池）中运行，如http-8081-Processor22，http-8081-Processor23
Count被这三个doGet方法共享，并且并行修改。
上面的结果，违反了线程安全的两个方面。
那么，我们怎样保证按照我们期望的结果运行呢？首先，我想保证产生的count都是顺序执行的。
我们将Servlet代码重构如下：
public class SimpleServlet extends HttpServlet
{
//A variable that is NOT thread-safe!
private int counter = 0;
private String mutex = "";
public void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
doPost(req, resp);
}
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
resp.getWriter().println("<HTML><BODY>");
resp.getWriter().println(this + ": <br>");
synchronized (mutex)
{
for (int c = 0; c < 10; c++)
{
resp.getWriter().println("Counter = " + counter + "<BR>");
try
{
Thread.sleep((long) Math.random() * 1000);
counter++;
}
catch (InterruptedException exc) { }
}
}
resp.getWriter().println("</BODY></HTML>");
}
}
com.zwchen.servlet.SimpleServlet@109da93:
Counter = 0
Counter = 1
Counter = 2
Counter = 3
Counter = 4
Counter = 5
Counter = 6
Counter = 7
Counter = 8
Counter = 9
com.zwchen.servlet.SimpleServlet@109da93:
Counter = 10
Counter = 11
Counter = 12
Counter = 13
Counter = 14
Counter = 15
Counter = 16
Counter = 17
Counter = 18
Counter = 19
com.zwchen.servlet.SimpleServlet@109da93:
Counter = 20
Counter = 21
Counter = 22
Counter = 23
Counter = 24
Counter = 25
Counter = 26
Counter = 27
Counter = 28
Counter = 29
这符合了我们的要求，输出都是按顺序的，这正是synchronized的含义。
原文地址：http://blog.csdn.net/victor_cindy1/article/details/45458845
