javax.el.ExpressionFactory Maven / Gradle / Ivy
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package javax.el;
/**
* Parses a String into a {@link ValueExpression} or
* {@link MethodExpression} instance for later evaluation.
*
* Classes that implement the EL expression language expose their
* functionality via this abstract class.
* The {@link #newInstance} method can be used to obtain an
* instance of the implementation.
* Technologies such as
* JavaServer Pages and JavaServer Faces provide access to an
* implementation via factory methods.
*
* The {@link #createValueExpression} method is used to parse expressions
* that evaluate to values (both l-values and r-values are supported).
* The {@link #createMethodExpression} method is used to parse expressions
* that evaluate to a reference to a method on an object.
*
* Unlike previous incarnations of this API, there is no way to parse
* and evaluate an expression in one single step. The expression needs to first
* be parsed, and then evaluated.
*
* Resolution of model objects is performed at evaluation time, via the
* {@link ELResolver} associated with the {@link ELContext} passed to
* the ValueExpression or MethodExpression.
*
* The ELContext object also provides access to the {@link FunctionMapper}
* and {@link VariableMapper} to be used when parsing the expression.
* EL function and variable mapping is performed at parse-time, and
* the results are
* bound to the expression. Therefore, the {@link ELContext},
* {@link FunctionMapper},
* and {@link VariableMapper}
* are not stored for future use and do not have to be
* Serializable.
*
* The createValueExpression and
* createMethodExpression methods must be thread-safe. That is,
* multiple threads may call these methods on the same
* ExpressionFactory object simultaneously. Implementations
* should synchronize access if they depend on transient state.
* Implementations should not, however, assume that only one object of
* each ExpressionFactory type will be instantiated; global
* caching should therefore be static.
*
* The ExpressionFactory must be able to handle the following
* types of input for the expression parameter:
*
* - Single expressions using the
${} delimiter
* (e.g. "${employee.lastName}").
* - Single expressions using the
#{} delimiter
* (e.g. "#{employee.lastName}").
* - Literal text containing no
${} or #{}
* delimiters (e.g. "John Doe").
* - Multiple expressions using the same delimiter (e.g.
*
"${employee.firstName}${employee.lastName}" or
* "#{employee.firstName}#{employee.lastName}").
* - Mixed literal text and expressions using the same delimiter (e.g.
*
"Name: ${employee.firstName} ${employee.lastName}").
*
*
* The following types of input are illegal and must cause an
* {@link ELException} to be thrown:
*
* - Multiple expressions using different delimiters (e.g.
*
"${employee.firstName}#{employee.lastName}").
* - Mixed literal text and expressions using different delimiters(e.g.
*
"Name: ${employee.firstName} #{employee.lastName}").
*
*
* @since JSP 2.1
*/
import java.util.Properties;
public abstract class ExpressionFactory {
/**
* Creates a new instance of a ExpressionFactory.
* This method uses the following ordered lookup procedure to determine
* the ExpressionFactory implementation class to load:
*
* - Use the Services API (as detailed in the JAR specification).
* If a resource with the name of
*
META-INF/services/javax.el.ExpressionFactory exists,
* then its first line, if present, is used as the UTF-8 encoded name of
* the implementation class.
* - Use the properties file "lib/el.properties" in the JRE directory.
* If this file exists and it is readable by the
*
java.util.Properties.load(InputStream) method,
* and it contains an entry whose key is "javax.el.ExpressionFactory",
* then the value of that entry is used as the name of the
* implementation class.
* - Use the
javax.el.ExpressionFactory system property.
* If a system property with this name is defined, then its value is
* used as the name of the implementation class.
* - Use a platform default implementation.
*
*/
public static ExpressionFactory newInstance() {
return ExpressionFactory.newInstance(null);
}
/**
* Create a new instance of a ExpressionFactory, with
* optional properties.
* This method uses the same lookup procedure as the one used in
* newInstance().
*
*
* If the argument properties is not null, and if the
* implementation contains a constructor with a single parameter of
* type java.util.Properties, then the constructor is used
* to create the instance.
*
*
* Properties are optional and can be ignored by an implementation.
*
* The name of a property should start with "javax.el."
*
* The following are some suggested names for properties.
*
* - javax.el.cacheSize
*
*
* @param properties Properties passed to the implementation.
* If null, then no properties.
*/
public static ExpressionFactory newInstance(Properties properties) {
return (ExpressionFactory) FactoryFinder.find(
"javax.el.ExpressionFactory",
"com.sun.el.ExpressionFactoryImpl",
properties);
}
/**
* Parses an expression into a {@link ValueExpression} for later
* evaluation. Use this method for expressions that refer to values.
*
* This method should perform syntactic validation of the expression.
* If in doing so it detects errors, it should raise an
* ELException.
*
* @param context The EL context used to parse the expression.
* The FunctionMapper and VariableMapper
* stored in the ELContext
* are used to resolve functions and variables found in
* the expression. They can be null, in which case
* functions or variables are not supported for this expression.
* The object
* returned must invoke the same functions and access the same
* variable mappings
* regardless of whether
* the mappings in the provided FunctionMapper
* and VariableMapper instances
* change between calling
* ExpressionFactory.createValueExpression() and any
* method on ValueExpression.
*
* Note that within the EL, the ${} and #{} syntaxes are treated identically.
* This includes the use of VariableMapper and FunctionMapper at expression creation
* time. Each is invoked if not null, independent
* of whether the #{} or ${} syntax is used for the expression.
* @param expression The expression to parse
* @param expectedType The type the result of the expression
* will be coerced to after evaluation.
* @return The parsed expression
* @throws NullPointerException Thrown if expectedType is null.
* @throws ELException Thrown if there are syntactical errors in the
* provided expression.
*/
public abstract ValueExpression createValueExpression(
ELContext context,
String expression,
Class expectedType);
/**
* Creates a ValueExpression that wraps an object instance. This
* method can be used to pass any object as a ValueExpression. The
* wrapper ValueExpression is read only, and returns the wrapped
* object via its getValue() method, optionally coerced.
*
* @param instance The object instance to be wrapped.
* @param expectedType The type the result of the expression
* will be coerced to after evaluation. There will be no
* coercion if it is Object.class,
* @throws NullPointerException Thrown if expectedType is null.
*/
public abstract ValueExpression createValueExpression(
Object instance,
Class expectedType);
/**
* Parses an expression into a {@link MethodExpression} for later
* evaluation. Use this method for expressions that refer to methods.
*
*
* If the expression is a String literal, a MethodExpression
* is created, which when invoked, returns the String literal,
* coerced to expectedReturnType. An ELException is thrown if
* expectedReturnType is void or if the coercion of the String literal
* to the expectedReturnType yields an error (see Section "1.16 Type
* Conversion").
*
* This method should perform syntactic validation of the expression.
* If in doing so it detects errors, it should raise an
* ELException.
*
* @param context The EL context used to parse the expression.
* The FunctionMapper and VariableMapper
* stored in the ELContext
* are used to resolve functions and variables found in
* the expression. They can be null, in which
* case functions or variables are not supported for this expression.
* The object
* returned must invoke the same functions and access the same variable
* mappings
* regardless of whether
* the mappings in the provided FunctionMapper
* and VariableMapper instances
* change between calling
* ExpressionFactory.createMethodExpression() and any
* method on MethodExpression.
*
* Note that within the EL, the ${} and #{} syntaxes are treated identically.
* This includes the use of VariableMapper and FunctionMapper at expression creation
* time. Each is invoked if not null, independent
* of whether the #{} or ${} syntax is used for the expression.
*
* @param expression The expression to parse
* @param expectedReturnType The expected return type for the method
* to be found. After evaluating the expression, the
* MethodExpression must check that the return type of
* the actual method matches this type. Passing in a value of
* null indicates the caller does not care what the
* return type is, and the check is disabled.
* @param expectedParamTypes The expected parameter types for the method to
* be found. Must be an array with no elements if there are
* no parameters expected. It is illegal to pass null.
* @return The parsed expression
* @throws ELException Thrown if there are syntactical errors in the
* provided expression.
* @throws NullPointerException if paramTypes is null.
*/
public abstract MethodExpression createMethodExpression(
ELContext context,
String expression,
Class expectedReturnType,
Class[] expectedParamTypes);
/**
* Coerces an object to a specific type according to the
* EL type conversion rules.
*
* An ELException is thrown if an error results from
* applying the conversion rules.
*
*
* @param obj The object to coerce.
* @param targetType The target type for the coercion.
* @throws ELException thrown if an error results from applying the
* conversion rules.
*/
public abstract Object coerceToType(
Object obj,
Class targetType);
}