jspintro7.html

j2eePDF格式的电子书

</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp74217"> </a><div class="pSmartList1"><li><code class="cCode">pageContext</code> - The context for the JSP page. Provides access to various objects including:</li></div>
<div class="pSmartList2"><ul class="pSmartList2">
<a name="wp74133"> </a><div class="pSmartList2"><li><code class="cCode">servletContext</code> - The context for the JSP page's servlet and any Web components contained in the same application. See <a  href="Servlets10.html#wp64724">Accessing the Web Context</a>.</li></div>
<a name="wp74146"> </a><div class="pSmartList2"><li><code class="cCode">session</code> - The session object for the client. See <a  href="Servlets11.html#wp64744">Maintaining Client State</a>.</li></div>
<a name="wp74147"> </a><div class="pSmartList2"><li><code class="cCode">request</code> - The request triggering the execution of the JSP page. See <a  href="Servlets7.html#wp64433">Getting Information from Requests</a>.</li></div>
<a name="wp83079"> </a><div class="pSmartList2"><li><code class="cCode">response</code> - The response returned by the JSP page. See <a  href="Servlets7.html#wp64531">Constructing Responses</a>).</li></div>
</ul></div>
</ul></div>
<a name="wp74145"> </a><p class="pBody">
In addition, several implicit objects are available that allow easy access to the following objects:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp71050"> </a><div class="pSmartList1"><li><code class="cCode">param</code> - maps a request parameter name to a single value </li></div>
<a name="wp71051"> </a><div class="pSmartList1"><li><code class="cCode">paramValues</code> - maps a request parameter name to an array of values</li></div>
<a name="wp71052"> </a><div class="pSmartList1"><li><code class="cCode">header</code> - maps a request header name to a single value </li></div>
<a name="wp71053"> </a><div class="pSmartList1"><li><code class="cCode">headerValues</code> - maps a request header name to an array of values</li></div>
<a name="wp71054"> </a><div class="pSmartList1"><li><code class="cCode">cookie</code> - maps a cookie name to a single cookie</li></div>
<a name="wp71055"> </a><div class="pSmartList1"><li><code class="cCode">initParam</code> - maps a context initialization parameter name to a single value </li></div>
</ul></div>
<a name="wp74144"> </a><p class="pBody">
Finally, there are objects that allow access to the various scoped variables described in <a  href="Servlets5.html#wp64315">Using Scope Objects</a>.
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp74136"> </a><div class="pSmartList1"><li><code class="cCode">pageScope</code> - maps page-scoped variable names to their values </li></div>
<a name="wp74137"> </a><div class="pSmartList1"><li><code class="cCode">requestScope</code> - maps request-scoped variable names to their values</li></div>
<a name="wp74138"> </a><div class="pSmartList1"><li><code class="cCode">sessionScope</code> - maps session-scoped variable names to their values   </li></div>
<a name="wp74139"> </a><div class="pSmartList1"><li><code class="cCode">applicationScope</code> - maps application-scoped variable names to their values</li></div>
</ul></div>
<a name="wp77250"> </a><p class="pBody">
When an expression references one of these objects by name, the appropriate object is returned instead of the corresponding attribute. For example: <code class="cCode">${pageContext}</code> returns the <code class="cCode">PageContext</code> object, even if there is an existing <code class="cCode">pageContext</code> attribute containing some other value.
</p>
<a name="wp77280"> </a><h3 class="pHeading2">
Literals
</h3>
<a name="wp72167"> </a><p class="pBody">
The JSP expression language defines the following literals:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp71083"> </a><div class="pSmartList1"><li>Boolean: <code class="cCode">true</code> and <code class="cCode">false</code></li></div>
<a name="wp71084"> </a><div class="pSmartList1"><li>Integer: as in Java</li></div>
<a name="wp71085"> </a><div class="pSmartList1"><li>Floating point: as in Java</li></div>
<a name="wp71086"> </a><div class="pSmartList1"><li>String: with single and double quotes. <code class="cCode">&quot;</code> is escaped as <code class="cCode">\&quot;</code>, &#39;<code class="cCode"> </code>is escaped as <code class="cCode">\</code>&#39;, and <code class="cCode">\</code> is escaped as <code class="cCode">\\</code>.</li></div>
<a name="wp71087"> </a><div class="pSmartList1"><li>Null: <code class="cCode">null</code></li></div>
</ul></div>
<a name="wp71088"> </a><h3 class="pHeading2">
Operators
</h3>
<a name="wp71089"> </a><p class="pBody">
In addition to the <code class="cCode">.</code> and <code class="cCode">[]</code> operators discussed in <a  href="JSPIntro7.html#wp84857">Variables</a>, the JSP expression language provides the following operators:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp71090"> </a><div class="pSmartList1"><li>Arithmetic: <code class="cCode">+</code>, <code class="cCode">- </code>(binary), <code class="cCode">*</code>, <code class="cCode">/</code> and <code class="cCode">div</code>, <code class="cCode">%</code> and <code class="cCode">mod</code>, <code class="cCode">-</code>(unary)</li></div>
<a name="wp71091"> </a><div class="pSmartList1"><li>Logical: <code class="cCode">and</code>, <code class="cCode">&amp;&amp;</code>, <code class="cCode">or</code>, <code class="cCode">||</code>, <code class="cCode">not</code>, <code class="cCode">!</code></li></div>
<a name="wp71092"> </a><div class="pSmartList1"><li>Relational: <code class="cCode">==</code>, <code class="cCode">eq</code>, <code class="cCode">!=</code>, <code class="cCode">ne</code>, <code class="cCode">&lt;</code>, <code class="cCode">lt</code>, <code class="cCode">&gt;</code>, <code class="cCode">gt</code>, <code class="cCode">&lt;=</code>, <code class="cCode">ge</code>, <code class="cCode">&gt;=</code>, <code class="cCode">le</code>. Comparisons may be made against other values, or against boolean, string, integer, or floating point literals.</li></div>
<a name="wp71093"> </a><div class="pSmartList1"><li>Empty: The <code class="cCode">empty</code> operator is a prefix operation that can be used to determine if a value is <code class="cCode">null</code> or empty.</li></div>
<a name="wp81442"> </a><div class="pSmartList1"><li>Conditional: <code class="cCode">A ? B : C</code>. Evaluate <code class="cCode">B</code> or <code class="cCode">C</code>, depending on the result of the evaluation of <code class="cCode">A</code>. </li></div>
</ul></div>
<a name="wp81311"> </a><p class="pBody">
The precedence of operators highest to lowest, left to right is:
</p>
<div class="pSmartList1"><ul class="pSmartList1">
<a name="wp81312"> </a><div class="pSmartList1"><li><code class="cCode">[] .</code></li></div>
<a name="wp81313"> </a><div class="pSmartList1"><li><code class="cCode">()</code> - Used to change the precedence of operators.</li></div>
<a name="wp81314"> </a><div class="pSmartList1"><li><code class="cCode">- </code>(unary)<code class="cCode"> not ! empty</code></li></div>
<a name="wp81316"> </a><div class="pSmartList1"><li><code class="cCode">* / div % mod</code></li></div>
<a name="wp81317"> </a><div class="pSmartList1"><li><code class="cCode">+ - </code>(binary)</li></div>
<a name="wp81318"> </a><div class="pSmartList1"><li><code class="cCode">&lt; &gt; &lt;= &gt;= lt gt le ge</code></li></div>
<a name="wp81319"> </a><div class="pSmartList1"><li><code class="cCode">== != eq ne</code></li></div>
<a name="wp81320"> </a><div class="pSmartList1"><li><code class="cCode">&amp;&amp; and</code></li></div>
<a name="wp81305"> </a><div class="pSmartList1"><li><code class="cCode">|| or</code></li></div>
<a name="wp81471"> </a><div class="pSmartList1"><li><code class="cCode">? :</code></li></div>
</ul></div>
<a name="wp71272"> </a><h3 class="pHeading2">
Reserved Words
</h3>
<a name="wp71273"> </a><p class="pBody">
The following words are reserved for the JSP expression language and should not be used as identifiers. 
</p>
<div class="pPreformattedRelative"><pre class="pPreformattedRelative">
and&nbsp;&nbsp;&nbsp;&nbsp;eq&nbsp;&nbsp;&nbsp;&nbsp;gt&nbsp;&nbsp;&nbsp;&nbsp;true&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;instanceof
or&nbsp;&nbsp;&nbsp;&nbsp;ne&nbsp;&nbsp;&nbsp;&nbsp;le&nbsp;&nbsp;&nbsp;&nbsp;false&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;empty
not&nbsp;&nbsp;&nbsp;&nbsp;lt&nbsp;&nbsp;&nbsp;&nbsp;ge&nbsp;&nbsp;&nbsp;&nbsp;null&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;div&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;mod<a name="wp71274"> </a>
</pre></div>
<a name="wp71275"> </a><p class="pBody">
Note that many of these words are not in the language now, but they may be in the future, so you should avoid using them. 
</p>
<a name="wp77076"> </a><h3 class="pHeading2">
Examples
</h3>
<a name="wp77077"> </a><p class="pBody">
<a  href="JSPIntro7.html#wp77083">Table 12-2</a> contains example EL expressions and the result of evaluating the expressions.</p><div align="left">
<table border="1" summary="Example Expressions" id="wp77083">
  <caption><a name="wp77083"> </a><div class="pTableTitle">
Table 12-2   Example Expressions&nbsp;
</div>
</caption>
  <tr align="center">    <th><a name="wp77087"> </a><div class="pCellHeading">
EL Expression
</div>
</th>
    <th><a name="wp77089"> </a><div class="pCellHeading">
Result
</div>
</th>
</tr>
  <tr align="left">    <td><a name="wp77091"> </a><div class="pCellBody">
<code class="cCode">${1 &gt; (4/2)}</code>
</div>
</td>
    <td><a name="wp77093"> </a><div class="pCellBody">
<code class="cCode">false</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77095"> </a><div class="pCellBody">
<code class="cCode">${4.0 &gt;= 3}</code>
</div>
</td>
    <td><a name="wp77097"> </a><div class="pCellBody">
<code class="cCode">true</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77099"> </a><div class="pCellBody">
<code class="cCode">${100.0 == 100}</code>
</div>
</td>
    <td><a name="wp77101"> </a><div class="pCellBody">
<code class="cCode">true</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77103"> </a><div class="pCellBody">
<code class="cCode">${(10*10) ne 100} </code>
</div>
</td>
    <td><a name="wp77105"> </a><div class="pCellBody">
<code class="cCode">false</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77107"> </a><div class="pCellBody">
<code class="cCode">${</code>&#39;<code class="cCode">a</code>&#39;<code class="cCode"> &lt; </code>&#39;<code class="cCode">b</code>&#39;<code class="cCode">}</code>
</div>
</td>
    <td><a name="wp77109"> </a><div class="pCellBody">
<code class="cCode">true</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77111"> </a><div class="pCellBody">
<code class="cCode">${</code>&#39;<code class="cCode">hip</code>&#39;<code class="cCode"> gt </code>&#39;<code class="cCode">hit</code>&#39;<code class="cCode">}</code>
</div>
</td>
    <td><a name="wp77113"> </a><div class="pCellBody">
<code class="cCode">false</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77115"> </a><div class="pCellBody">
<code class="cCode">${4 &gt; 3}</code>
</div>
</td>
    <td><a name="wp77117"> </a><div class="pCellBody">
<code class="cCode">true</code>
</div>
</td>
</tr>
  <tr align="left">    <td><a name="wp77119"> </a><div class="pCellBody">
<code class="cCode">${1.2E4 + 1.4}</code>
</div>
</td>
    <td><a name="wp77121"> </a><div class="pCellBody">
<code class="cCode">12001.4</code>
</div>
</td>
</tr>