Expression (Java EE 5)

java.lang.Object
  javax.el.Expression

表达式子类 ValueExpression 和 MethodExpression 的基类，它实现两者共同的特性。

所有表达式必须实现 equals() 和 hashCode() 方法，以便可以比较两个表达式的相等性。在此类中它们被重定义为抽象方法，以在子类中强制实现。

所有表达式还必须是 Serializable，以便它们可以保存和存储。

Expression 还设计为不可改变，从而只需要为任何给定的表达式 String/FunctionMapper 创建一个实例。这允许容器预创建表达式，而且无需在每次对其求值时重新解析它们。

since

JSP 2.1

英文文档：

Base class for the expression subclasses ValueExpression and MethodExpression, implementing characterstics common to both.

All expressions must implement the equals() and hashCode() methods so that two expressions can be compared for equality. They are redefined abstract in this class to force their implementation in subclasses.

All expressions must also be Serializable so that they can be saved and restored.

Expressions are also designed to be immutable so that only one instance needs to be created for any given expression String / FunctionMapper. This allows a container to pre-create expressions and not have to re-parse them each time they are evaluated.

Since:

JSP 2.1

See Also:

Serialized Form

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Constructor Summary
Expression()

 

Method Summary
abstract  boolean	equals(Object obj)           Determines whether the specified object is equal to this Expression.
abstract  String	getExpressionString()           Returns the original String used to create this Expression, unmodified.
abstract  int	hashCode()           Returns the hash code for this Expression.
abstract  boolean	isLiteralText()           Returns whether this expression was created from only literal text.

 

Methods inherited from class java.lang.Object
clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

 

Constructor Detail

public Expression()

英文文档：

Expression

public Expression()

Method Detail

abstract public String getExpressionString()
返回用于创建此 Expression 的原始 String（未经修改）。

此方法不仅仅用于调试目的，而且还用于比较（例如，为了确保配置文件中的表达式尚未更改）。

此方法没有提供重新创建表达式的足够信息。两个不同的表达式可以有完全相同的表达式字符串，不同的函数映射。应该使用序列化来保存和存储 Expression 的状态。

return

原始表达式 String。

英文文档：

getExpressionString

public abstract String getExpressionString()

Returns the original String used to create this Expression, unmodified.

This is used for debugging purposes but also for the purposes of comparison (e.g. to ensure the expression in a configuration file has not changed).

This method does not provide sufficient information to re-create an expression. Two different expressions can have exactly the same expression string but different function mappings. Serialization should be used to save and restore the state of an Expression.

Returns:

The original expression String.

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abstract public boolean equals(Object obj)
确定指定的对象是否等于此 Expression。

当且仅当参数不是 null 而是相同类型（ValueExpression 或 MethodExpression）的 Expression 对象，且解析的表示形式相同时，结果为 true。

注意，即使两个表达式的表达式 String 不同，这两个表达式也可以相等。例如，如果相应的 FunctionMapper 将 fn1:foo 和 fn2:foo 映射到同一方法，则 ${fn1:foo()} 和 ${fn2:foo()} 相等。

obj	要测试相等性的 Object。
return	如果 obj 等于此 Expression，则返回 true；否则返回 false。
See also	java.util.Hashtable, equals(java.lang.Object)

英文文档：

equals

public abstract boolean equals(Object obj)

Determines whether the specified object is equal to this Expression.

The result is true if and only if the argument is not null, is an Expression object that is the of the same type (ValueExpression or MethodExpression), and has an identical parsed representation.

Note that two expressions can be equal if their expression Strings are different. For example, ${fn1:foo()} and ${fn2:foo()} are equal if their corresponding FunctionMappers mapped fn1:foo and fn2:foo to the same method.

Overrides:

equals in class Object

Parameters:

obj - the Object to test for equality.

Returns:

true if obj equals this Expression; false otherwise.

See Also:

Hashtable, Object.equals(java.lang.Object)

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abstract public int hashCode()
返回此 Expression 的哈希码。

有关两个表达式的表达式 String 不同时如何才能相等，请参阅 #equals 方法中的说明。记住，如果根据 equals(Object) 方法，两个对象是相等的，那么对这两个对象中的每个对象调用 hashCode 方法都必须生成相同的整数结果。必须特别注意实现并且正确实现 hashCode。

return	此 Expression 的哈希码。
See also	equals, java.util.Hashtable, hashCode()

英文文档：

hashCode

public abstract int hashCode()

Returns the hash code for this Expression.

See the note in the equals(java.lang.Object) method on how two expressions can be equal if their expression Strings are different. Recall that if two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. Implementations must take special note and implement hashCode correctly.

Overrides:

hashCode in class Object

Returns:

The hash code for this Expression.

See Also:

equals(java.lang.Object), Hashtable, Object.hashCode()

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abstract public boolean isLiteralText()
返回此表达式是否仅根据字面值文本创建。

当且仅当此表达式的表达式字符串是根据不包含非转义的 EL 分隔符（${...} 或 #{...}）创建时，该方法才返回 true。

return

如果此表达式是根据字面值文本创建的，则返回 true；否则返回 false。

英文文档：

isLiteralText

public abstract boolean isLiteralText()

Returns whether this expression was created from only literal text.

This method must return true if and only if the expression string this expression was created from contained no unescaped EL delimeters (${...} or #{...}).

Returns:

true if this expression was created from only literal text; false otherwise.

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