valueexpression.html
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><!--NewPage--><HTML><HEAD><!-- Generated by javadoc (build 1.5.0_14) on Mon Jan 28 05:47:05 PST 2008 --><TITLE>ValueExpression (Java EE 5)</TITLE><META NAME="keywords" CONTENT="javax.el.ValueExpression class"><LINK REL ="stylesheet" TYPE="text/css" HREF="../../stylesheet.css" TITLE="Style"><SCRIPT type="text/javascript">function windowTitle(){ parent.document.title="ValueExpression (Java EE 5)";}</SCRIPT><NOSCRIPT></NOSCRIPT></HEAD><BODY BGCOLOR="white" onload="windowTitle();"><!-- ========= START OF TOP NAVBAR ======= --><A NAME="navbar_top"><!-- --></A><A HREF="#skip-navbar_top" title="Skip navigation links"></A><TABLE BORDER="0" WIDTH="100%" CELLPADDING="1" CELLSPACING="0" SUMMARY=""><TR><TD COLSPAN=2 BGCOLOR="#EEEEFF" CLASS="NavBarCell1"><A NAME="navbar_top_firstrow"><!-- --></A><TABLE BORDER="0" CELLPADDING="0" CELLSPACING="3" SUMMARY=""> <TR ALIGN="center" VALIGN="top"> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../overview-summary.html"><FONT CLASS="NavBarFont1"><B>Overview</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-summary.html"><FONT CLASS="NavBarFont1"><B>Package</B></FONT></A> </TD> <TD BGCOLOR="#FFFFFF" CLASS="NavBarCell1Rev"> <FONT CLASS="NavBarFont1Rev"><B>Class</B></FONT> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-tree.html"><FONT CLASS="NavBarFont1"><B>Tree</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../deprecated-list.html"><FONT CLASS="NavBarFont1"><B>Deprecated</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../index-all.html"><FONT CLASS="NavBarFont1"><B>Index</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../help-doc.html"><FONT CLASS="NavBarFont1"><B>Help</B></FONT></A> </TD> </TR></TABLE></TD><TD ALIGN="right" VALIGN="top" ROWSPAN=3><EM></EM></TD></TR><TR><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../javax/el/ResourceBundleELResolver.html" title="class in javax.el"><B>PREV CLASS</B></A> <A HREF="../../javax/el/VariableMapper.html" title="class in javax.el"><B>NEXT CLASS</B></A></FONT></TD><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../index.html?javax/el/ValueExpression.html" target="_top"><B>FRAMES</B></A> <A HREF="ValueExpression.html" target="_top"><B>NO FRAMES</B></A> <SCRIPT type="text/javascript"> <!-- if(window==top) { document.writeln('<A HREF="../../allclasses-noframe.html"><B>All Classes</B></A>'); } //--></SCRIPT><NOSCRIPT> <A HREF="../../allclasses-noframe.html"><B>All Classes</B></A></NOSCRIPT></FONT></TD></TR><TR><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2"> SUMMARY: NESTED | FIELD | <A HREF="#constructor_summary">CONSTR</A> | <A HREF="#method_summary">METHOD</A></FONT></TD><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2">DETAIL: FIELD | <A HREF="#constructor_detail">CONSTR</A> | <A HREF="#method_detail">METHOD</A></FONT></TD></TR></TABLE><A NAME="skip-navbar_top"></A><!-- ========= END OF TOP NAVBAR ========= --><HR><!-- ======== START OF CLASS DATA ======== --><H2><FONT SIZE="-1">javax.el</FONT><BR>Class ValueExpression</H2><PRE><A HREF="http://java.sun.com/j2se/1.5/docs/api/java/lang/Object.html" title="class or interface in java.lang">java.lang.Object</A> <IMG SRC="../../resources/inherit.gif" ALT="extended by "><A HREF="../../javax/el/Expression.html" title="class in javax.el">javax.el.Expression</A> <IMG SRC="../../resources/inherit.gif" ALT="extended by "><B>javax.el.ValueExpression</B></PRE><DL><DT><B>All Implemented Interfaces:</B> <DD><A HREF="http://java.sun.com/j2se/1.5/docs/api/java/io/Serializable.html" title="class or interface in java.io">Serializable</A></DD></DL><HR><DL><DT><PRE>public abstract class <B>ValueExpression</B><DT>extends <A HREF="../../javax/el/Expression.html" title="class in javax.el">Expression</A></DL></PRE><P>An <code>Expression</code> that can get or set a value. <p>In previous incarnations of this API, expressions could only be read. <code>ValueExpression</code> objects can now be used both to retrieve a value and to set a value. Expressions that can have a value set on them are referred to as l-value expressions. Those that cannot are referred to as r-value expressions. Not all r-value expressions can be used as l-value expressions (e.g. <code>"${1+1}"</code> or <code>"${firstName} ${lastName}"</code>). See the EL Specification for details. Expressions that cannot be used as l-values must always return <code>true</code> from <code>isReadOnly()</code>.</p> <p>The <code><A HREF="../../javax/el/ExpressionFactory.html#createValueExpression(javax.el.ELContext, java.lang.String, java.lang.Class)"><CODE>ExpressionFactory.createValueExpression(javax.el.ELContext, java.lang.String, java.lang.Class<?>)</CODE></A></code> method can be used to parse an expression string and return a concrete instance of <code>ValueExpression</code> that encapsulates the parsed expression. The <A HREF="../../javax/el/FunctionMapper.html" title="class in javax.el"><CODE>FunctionMapper</CODE></A> is used at parse time, not evaluation time, so one is not needed to evaluate an expression using this class. However, the <A HREF="../../javax/el/ELContext.html" title="class in javax.el"><CODE>ELContext</CODE></A> is needed at evaluation time.</p> <p>The <A HREF="../../javax/el/ValueExpression.html#getValue(javax.el.ELContext)"><CODE>getValue(javax.el.ELContext)</CODE></A>, <A HREF="../../javax/el/ValueExpression.html#setValue(javax.el.ELContext, java.lang.Object)"><CODE>setValue(javax.el.ELContext, java.lang.Object)</CODE></A>, <A HREF="../../javax/el/ValueExpression.html#isReadOnly(javax.el.ELContext)"><CODE>isReadOnly(javax.el.ELContext)</CODE></A> and <A HREF="../../javax/el/ValueExpression.html#getType(javax.el.ELContext)"><CODE>getType(javax.el.ELContext)</CODE></A> methods will evaluate the expression each time they are called. The <A HREF="../../javax/el/ELResolver.html" title="class in javax.el"><CODE>ELResolver</CODE></A> in the <code>ELContext</code> is used to resolve the top-level variables and to determine the behavior of the <code>.</code> and <code>[]</code> operators. For any of the four methods, the <A HREF="../../javax/el/ELResolver.html#getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)"><CODE>ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)</CODE></A> method is used to resolve all properties up to but excluding the last one. This provides the <code>base</code> object. At the last resolution, the <code>ValueExpression</code> will call the corresponding <A HREF="../../javax/el/ELResolver.html#getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)"><CODE>ELResolver.getValue(javax.el.ELContext, java.lang.Object, java.lang.Object)</CODE></A>, <A HREF="../../javax/el/ELResolver.html#setValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)"><CODE>ELResolver.setValue(javax.el.ELContext, java.lang.Object, java.lang.Object, java.lang.Object)</CODE></A>, <A HREF="../../javax/el/ELResolver.html#isReadOnly(javax.el.ELContext, java.lang.Object, java.lang.Object)"><CODE>ELResolver.isReadOnly(javax.el.ELContext, java.lang.Object, java.lang.Object)</CODE></A> or <A HREF="../../javax/el/ELResolver.html#getType(javax.el.ELContext, java.lang.Object, java.lang.Object)"><CODE>ELResolver.getType(javax.el.ELContext, java.lang.Object, java.lang.Object)</CODE></A> method, depending on which was called on the <code>ValueExpression</code>. </p> <p>See the notes about comparison, serialization and immutability in the <A HREF="../../javax/el/Expression.html" title="class in javax.el"><CODE>Expression</CODE></A> javadocs.<P><P><DL><DT><B>Since:</B></DT> <DD>JSP 2.1</DD><DT><B>See Also:</B><DD><A HREF="../../javax/el/ELResolver.html" title="class in javax.el"><CODE>ELResolver</CODE></A>, <A HREF="../../javax/el/Expression.html" title="class in javax.el"><CODE>Expression</CODE></A>, <A HREF="../../javax/el/ExpressionFactory.html" title="class in javax.el"><CODE>ExpressionFactory</CODE></A>, <A HREF="../../serialized-form.html#javax.el.ValueExpression">Serialized Form</A></DL><HR><P><!-- ======== CONSTRUCTOR SUMMARY ======== --><A NAME="constructor_summary"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"><B>Constructor Summary</B></FONT></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#ValueExpression()">ValueExpression</A></B>()</CODE><BR> </TD></TR></TABLE> <!-- ========== METHOD SUMMARY =========== --><A NAME="method_summary"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"><B>Method Summary</B></FONT></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>abstract <A HREF="http://java.sun.com/j2se/1.5/docs/api/java/lang/Class.html" title="class or interface in java.lang">Class</A><?></CODE></FONT></TD><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#getExpectedType()">getExpectedType</A></B>()</CODE><BR> Returns the type the result of the expression will be coerced to after evaluation.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>abstract <A HREF="http://java.sun.com/j2se/1.5/docs/api/java/lang/Class.html" title="class or interface in java.lang">Class</A><?></CODE></FONT></TD><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#getType(javax.el.ELContext)">getType</A></B>(<A HREF="../../javax/el/ELContext.html" title="class in javax.el">ELContext</A> context)</CODE><BR> Evaluates the expression relative to the provided context, and returns the most general type that is acceptable for an object to be passed as the <code>value</code> parameter in a future call to the <A HREF="../../javax/el/ValueExpression.html#setValue(javax.el.ELContext, java.lang.Object)"><CODE>setValue(javax.el.ELContext, java.lang.Object)</CODE></A> method.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>abstract <A HREF="http://java.sun.com/j2se/1.5/docs/api/java/lang/Object.html" title="class or interface in java.lang">Object</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#getValue(javax.el.ELContext)">getValue</A></B>(<A HREF="../../javax/el/ELContext.html" title="class in javax.el">ELContext</A> context)</CODE><BR> Evaluates the expression relative to the provided context, and returns the resulting value.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>abstract boolean</CODE></FONT></TD><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#isReadOnly(javax.el.ELContext)">isReadOnly</A></B>(<A HREF="../../javax/el/ELContext.html" title="class in javax.el">ELContext</A> context)</CODE><BR> Evaluates the expression relative to the provided context, and returns <code>true</code> if a call to <A HREF="../../javax/el/ValueExpression.html#setValue(javax.el.ELContext, java.lang.Object)"><CODE>setValue(javax.el.ELContext, java.lang.Object)</CODE></A> will always fail.</TD></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>abstract void</CODE></FONT></TD><TD><CODE><B><A HREF="../../javax/el/ValueExpression.html#setValue(javax.el.ELContext, java.lang.Object)">setValue</A></B>(<A HREF="../../javax/el/ELContext.html" title="class in javax.el">ELContext</A> context, <A HREF="http://java.sun.com/j2se/1.5/docs/api/java/lang/Object.html" title="class or interface in java.lang">Object</A> value)</CODE><BR> Evaluates the expression relative to the provided context, and sets the result to the provided value.</TD></TR></TABLE> <A NAME="methods_inherited_from_class_javax.el.Expression"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor"><TH ALIGN="left"><B>Methods inherited from class javax.el.<A HREF="../../javax/el/Expression.html" title="class in javax.el">Expression</A></B></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><A HREF="../../javax/el/Expression.html#equals(java.lang.Object)">equals</A>, <A HREF="../../javax/el/Expression.html#getExpressionString()">getExpressionString</A>, <A HREF="../../javax/el/Expression.html#hashCode()">hashCode</A>, <A HREF="../../javax/el/Expression.html#isLiteralText()">isLiteralText</A></CODE></TD></TR></TABLE>⌨️ 快捷键说明
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