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📁 java安全－论证授权
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>JAASExample 操作流</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 7 页（共7 页）</nobr></font></td>
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<p>下面是由 JAASExample 图说明的总体认证与授权流的简要描述。以下每个步骤将在本教程的其它地方进行更为详细的描述。</p>
<p>我们从认证的第一步开始，就是要创建登录环境并试图登录。<code>LoginContext</code>
是一个 Java 类，它使用 login.config 文件中的信息来决定要调用哪些登录模块以及将使用什么标准来确定是否成功。
对本示例，有两个登录模块。
第一个登录模块是 <code>AlwaysLoginModule</code>，它不需要密码，所以它总是成功的（这是不切实际的，但它足以说明 JAAS 是如何工作的）。
该模块用关键字 <code>required</code> 标记，表示它是成功所必需的（它总是成功）。第二个登录模块是<code>PasswordLoginModule</code>，它需要密码，但该模块的成功与否是可选的，因为它用关键字
<code>optional</code> 标记。这表示即使 <code>PasswordLoginModule</code> 失败，但总体登录仍可成功。</p>
<p>初始化之后，选择的登录模块经历由 <code>LoginContext</code> 控制的两阶段提交过程。
作为该过程的一部分，调用 <code>UsernamePasswordCallbackHandler</code> 以获取个人（用
<code>Subject</code> 对象表示）的用户名和密码。如果认证成功，则 <code>Principal</code>
被添加到 <code>Subject</code> 中。<code>Subject</code> 可以有许多 <code>Principal</code>（在该示例中，是&ldquo;Brad&rdquo;和&ldquo;joeuser&rdquo;），每个 <code>Principal</code> 都授予用户对系统的不同级别的访问权。这样就完成了认证步骤。</p>
<p>一旦认证完成，通过使用程序认证技术和 <code>doAs</code> 方法，用 <code>Subject</code> 来尝试执行一些敏感的工资单操作代码。JAAS 检查是否授予 <code>Subject</code> 访问权。
如果 <code>Subject</code> 有一个授权访问工资单代码的 <code>Principal</code>，
那么允许继续执行。否则，将拒绝执行。</p>
<p>接下来，我们尝试使用声明性授权技术和
<code>doAsPrivilaged</code> 方法来执行一些敏感的职员信息操作代码。这次，JAAS 部署用户定义的特权（<code>PersonnelPermission</code>）、Java 策略文件（jaas.policy）和 Java 访问控制器（<code>AccessController</code>）用来决定是否可以继续执行。</p>
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