org.camunda.bpm.engine.impl.javax.el

Class ExpressionFactory

java.lang.Object

org.camunda.bpm.engine.impl.javax.el.ExpressionFactory

Direct Known Subclasses:

ExpressionFactoryImpl

public abstract class ExpressionFactory
extends Object

Parses a String into a ValueExpression or MethodExpression instance for later evaluation. Classes that implement the EL expression language expose their functionality via this abstract class. There is no concrete implementation of this API available in this package. Technologies such as JavaServer Pages and JavaServer Faces provide access to an implementation via factory methods. The createValueExpression(ELContext, String, Class) method is used to parse expressions that evaluate to values (both l-values and r-values are supported). The createMethodExpression(ELContext, String, Class, Class[]) 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 ELResolver associated with the ELContext passed to the ValueExpression or MethodExpression. The ELContext object also provides access to the FunctionMapper and 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 ELContext, FunctionMapper, and 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 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}").

Constructor Summary

Constructors

Constructor and Description
ExpressionFactory()

Method Summary

Methods

Modifier and Type	Method and Description
abstract Object	coerceToType(Object obj, Class<?> targetType) Coerces an object to a specific type according to the EL type conversion rules.
abstract MethodExpression	createMethodExpression(ELContext context, String expression, Class<?> expectedReturnType, Class<?>[] expectedParamTypes) Parses an expression into a MethodExpression for later evaluation.
abstract ValueExpression	createValueExpression(ELContext context, String expression, Class<?> expectedType) Parses an expression into a ValueExpression for later evaluation.
abstract ValueExpression	createValueExpression(Object instance, Class<?> expectedType) Creates a ValueExpression that wraps an object instance.
static ExpressionFactory	newInstance() Creates a new instance of a ExpressionFactory.
static ExpressionFactory	newInstance(Properties properties) Create a new instance of a ExpressionFactory, with optional properties.

Methods inherited from class java.lang.Object

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

Constructor Detail

ExpressionFactory

public ExpressionFactory()

Method Detail

newInstance

public static ExpressionFactory newInstance()

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.

Returns:

An instance of ExpressionFactory.

Throws:

ELException - if a factory class cannot be found or instantiation fails.

newInstance

public static ExpressionFactory newInstance(Properties properties)

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

Parameters:

properties - Properties passed to the constructor of the implementation.

Returns:

An instance of ExpressionFactory.

Throws:

ELException - if a factory class cannot be found or instantiation fails.

coerceToType

public abstract Object coerceToType(Object obj,
                  Class<?> targetType)

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.

Parameters:

obj - The object to coerce.

targetType - The target type for the coercion.

Returns:

the coerced object

Throws:

ELException - if an error results from applying the conversion rules.

createMethodExpression

public abstract MethodExpression createMethodExpression(ELContext context,
                                      String expression,
                                      Class<?> expectedReturnType,
                                      Class<?>[] expectedParamTypes)

Parses an expression into a 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.

Parameters:

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.

expression - The expression to parse

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.

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.

Returns:

The parsed expression

Throws:

ELException - Thrown if there are syntactical errors in the provided expression.

NullPointerException - if paramTypes is null.

createValueExpression

public abstract ValueExpression createValueExpression(ELContext context,
                                    String expression,
                                    Class<?> expectedType)

Parses an expression into a 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.

Parameters:

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.

expression - The expression to parse

expectedType - The type the result of the expression will be coerced to after evaluation.

Returns:

The parsed expression

Throws:

ELException - Thrown if there are syntactical errors in the provided expression.

NullPointerException - if paramTypes is null.

createValueExpression

public abstract ValueExpression createValueExpression(Object instance,
                                    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.

Parameters:

instance - The object instance to be wrapped.

expectedType - The type the result of the expression will be coerced to after evaluation. There will be no coercion if it is Object.class,

Returns:

a ValueExpression that wraps the given object instance.