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VariableInfo (JavaServer Pages API Documentation)

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javax.servlet.jsp.tagext
Class VariableInfo

java.lang.Object
  javax.servlet.jsp.tagext.VariableInfo

public class VariableInfo
extends java.lang.Object

Information on the scripting variables that are created/modified by a tag (at run-time). This information is provided by TagExtraInfo classes and it is used by the translation phase of JSP.

Scripting variables generated by a custom action have an associated scope of either AT_BEGIN, NESTED, or AT_END.

The class name (VariableInfo.getClassName) in the returned objects is used to determine the types of the scripting variables. Note that because scripting variables are assigned their values from scoped attributes which cannot be of primitive types, "boxed" types such as java.lang.Integer must be used instead of primitives.

The class name may be a Fully Qualified Class Name, or a short class name.

If a Fully Qualified Class Name is provided, it should refer to a class that should be in the CLASSPATH for the Web Application (see Servlet 2.4 specification - essentially it is WEB-INF/lib and WEB-INF/classes). Failure to be so will lead to a translation-time error.

If a short class name is given in the VariableInfo objects, then the class name must be that of a public class in the context of the import directives of the page where the custom action appears. The class must also be in the CLASSPATH for the Web Application (see Servlet 2.4 specification - essentially it is WEB-INF/lib and WEB-INF/classes). Failure to be so will lead to a translation-time error.

Usage Comments

Frequently a fully qualified class name will refer to a class that is known to the tag library and thus, delivered in the same JAR file as the tag handlers. In most other remaining cases it will refer to a class that is in the platform on which the JSP processor is built (like J2EE). Using fully qualified class names in this manner makes the usage relatively resistant to configuration errors.

A short name is usually generated by the tag library based on some attributes passed through from the custom action user (the author), and it is thus less robust: for instance a missing import directive in the referring JSP page will lead to an invalid short name class and a translation error.

Synchronization Protocol

The result of the invocation on getVariableInfo is an array of VariableInfo objects. Each such object describes a scripting variable by providing its name, its type, whether the variable is new or not, and what its scope is. Scope is best described through a picture:

NESTED, AT_BEGIN and AT_END Variable Scopes

The JSP 2.0 specification defines the interpretation of 3 values:

NESTED, if the scripting variable is available between the start tag and the end tag of the action that defines it.
AT_BEGIN, if the scripting variable is available from the start tag of the action that defines it until the end of the scope.
AT_END, if the scripting variable is available after the end tag of the action that defines it until the end of the scope.

The scope value for a variable implies what methods may affect its value and thus where synchronization is needed as illustrated by the table below. Note: the synchronization of the variable(s) will occur after the respective method has been called.

Variable Synchronization Points

doStartTag() doInitBody() doAfterBody() doEndTag() doTag()

Tag
AT_BEGIN, NESTED

AT_BEGIN, AT_END

IterationTag
AT_BEGIN, NESTED

AT_BEGIN, NESTED
AT_BEGIN, AT_END

BodyTag
AT_BEGIN, NESTED¹
AT_BEGIN, NESTED¹
AT_BEGIN, NESTED
AT_BEGIN, AT_END

SimpleTag

AT_BEGIN, AT_END

¹ Called after doStartTag() if EVAL_BODY_INCLUDE is returned, or after doInitBody() otherwise.

Variable Information in the TLD

Scripting variable information can also be encoded directly for most cases into the Tag Library Descriptor using the <variable> subelement of the <tag> element. See the JSP specification.

Field Summary

static int AT_BEGIN
          Scope information that scripting variable is visible after start tag.

static int AT_END
          Scope information that scripting variable is visible after end tag.

static int NESTED
          Scope information that scripting variable is visible only within the start/end tags.

Constructor Summary

VariableInfo(java.lang.String varName, java.lang.String className, boolean declare, int scope)
          Constructor These objects can be created (at translation time) by the TagExtraInfo instances.

Method Summary

java.lang.String getClassName()
          Returns the type of this variable.

boolean getDeclare()
          Returns whether this is a new variable.

int getScope()
          Returns the lexical scope of the variable.

java.lang.String getVarName()
          Returns the name of the scripting variable.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

NESTED

public static final int NESTED

Scope information that scripting variable is visible only within the start/end tags.

See Also:: Constant Field Values

AT_BEGIN

public static final int AT_BEGIN

Scope information that scripting variable is visible after start tag.

See Also:: Constant Field Values

AT_END

public static final int AT_END

Scope information that scripting variable is visible after end tag.

See Also:: Constant Field Values

Constructor Detail

VariableInfo

public VariableInfo(java.lang.String varName,
                    java.lang.String className,
                    boolean declare,
                    int scope)

Constructor These objects can be created (at translation time) by the TagExtraInfo instances.
Parameters:: varName - The name of the scripting variable; className - The type of this variable; declare - If true, it is a new variable (in some languages this will require a declaration); scope - Indication on the lexical scope of the variable