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<a name="navskip"></a><td height="25" colspan="4"><img alt="8.开发线程安全的 servlet" src="imagemaster/titlebar8.jpg" border="0" height="25" width="562"></td>
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>示例问题</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr> 第 6 页（共6 页）</nobr></font></td>
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</p>
<font size="2" face="Verdana, Arial, Helvetica"><b>问题 1：</b>
<p>考虑下面的 servlet 代码：</p>
<pre>
<code style="font-family: Courier New, Courier, monospace; font-size: 12">
public class MyServlet extends HttpServlet
{
final static int i=0;
public void doGet(HttpServletRequest req, HttpServletResponse res)
{
private HttpSession session=req.getSession();
private ServletContext ctx=getServletContext();
synchronized(ctx)
{
Object obj=ctx.getAttribute();
// code to alter obj
}
}
}
</code>
</pre>
<p>上面代码中的哪些变量是线程安全的？</p>
<p>
<b>选择：</b>
</p>
<ul>
<li>
<b>A.</b> <code>i</code>
</li>
<li>
<b>B.</b> <code>session</code>
</li>
<li>
<b>C.</b> <code>ctx</code>
</li>
<li>
<b>D.</b> <code>req</code>
</li>
<li>
<b>E.</b> <code>obj</code>
</li>
<li>
<b>F.</b> <code>res</code>
</li>
</ul>
<p>
<b>正确答案：</b>
</p>
<ul>
<li>
<b>A</b>、<b>C</b>、<b>D</b> 和 <b>F</b>
</li>
</ul>
<p>
<b>说明：</b>
</p>
<p>静态变量 <code>i</code> 是线程安全的，因为它是 final（不能被修改），否则它将不是安全的。请求和响应对象的作用域只在请求的生命周期，因此它们也是线程安全的。会话和 <code>ServletContext</code> 对象可以从多个线程访问，同时处理多个请求，因此它们不是线程安全的。但在本例中，同步了 <code>ServletContext</code> 对象，因此它只能由一个线程一次访问。<code>obj</code> 不是线程安全的，因为即使同步了 <code>ServletContext</code> 对象，它的属性也没有同步。它们需要另外进行同步。因此，选项 B 和 E 是不正确的，而选项 A、C、D 和 F 是正确的。</p>
<p>
<b>问题 2：</b>
</p>
<p>下面哪些语句是正确的？</p>
<p>
<b>选择：</b>
</p>
<ul>
<li>
<b>A.</b> 可以在实现 <code>SingleThreadModel</code> 的 servlet 上创建多个实例</li>
<li>
<b>B.</b> 不止为实现 <code>SingleThreadModel</code> 的 servlet 创建一个实例</li>
<li>
<b>C.</b> 即使 <code>SingleThreadModel</code> servlet 中的静态变量也是线程安全的</li>
<li>
<b>D.</b> 如果没有实现线程模型，默认情况下，servlet 是在多线程模型中执行的。</li>
</ul>
<p>
<b>正确答案：</b>
</p>
<ul>
<li>
<b>A</b> 和 <b>D</b>
</li>
</ul>
<p>
<b>说明：</b>
</p>
<p>当实现了 <code>SingleThreadModel</code>，servlet 容器确保一次只有一个线程在执行 servlet 的方法。因此对于多个请求，将会发生什么？在那种情形中，容器可以实例化 servlet 的多个实例，用于处理多个请求，因此选项 A 是正确的，B 是不正确的。</p>
<p>如果没有实现 <code>SingleThreadModel</code> 接口，servlet 就使用多线程模型（也就是，多个线程可以访问 servlet 的方法）。静态变量可以通过同一个类的多个实例访问，因此它们始终不是线程安全的。因此选项 B 和 C 是不正确的，选项 A 和 D 是正确的。</p>
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