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/*
* Based on JUEL 2.2.1 code, 2006-2009 Odysseus Software GmbH
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.activiti.engine.impl.javax.el;
import java.beans.FeatureDescriptor;
import java.util.Iterator;
/**
* Enables customization of variable and property resolution behavior for EL expression evaluation.
*
* While evaluating an expression, the ELResolver associated with the ELContext is consulted to do
* the initial resolution of the first variable of an expression. It is also consulted when a . or
* [] operator is encountered, except for the last such operator in a method expression, in which
* case the resolution rules are hard coded.
*
* For example, in the EL expression ${employee.lastName}, the ELResolver determines what object
* employee refers to, and what it means to get the lastName property on that object.
*
* Most methods in this class accept a base and property parameter. In the case of variable
* resolution (e.g. determining what employee refers to in ${employee.lastName}), the base parameter
* will be null and the property parameter will always be of type String. In this case, if the
* property is not a String, the behavior of the ELResolver is undefined.
*
* In the case of property resolution, the base parameter identifies the base object and the
* property object identifies the property on that base. For example, in the expression
* ${employee.lastName}, base is the result of the variable resolution for employee and property is
* the string "lastName". In the expression ${y[x]}, base is the result of the variable resolution
* for y and property is the result of the variable resolution for x.
*
* Though only a single ELResolver is associated with an ELContext, there are usually multiple
* resolvers considered for any given variable or property resolution. ELResolvers are combined
* together using {@link CompositeELResolver}s, to define rich semantics for evaluating an
* expression. For the {@link #getValue(ELContext, Object, Object)},
* {@link #getType(ELContext, Object, Object)}, {@link #setValue(ELContext, Object, Object, Object)}
* and {@link #isReadOnly(ELContext, Object, Object)} methods, an ELResolver is not responsible for
* resolving all possible (base, property) pairs. In fact, most resolvers will only handle a base of
* a single type. To indicate that a resolver has successfully resolved a particular (base,
* property) pair, it must set the propertyResolved property of the ELContext to true. If it could
* not handle the given pair, it must leave this property alone. The caller must ignore the return
* value of the method if propertyResolved is false.
*
* The {@link #getFeatureDescriptors(ELContext, Object)} and
* {@link #getCommonPropertyType(ELContext, Object)} methods are primarily designed for design-time
* tool support, but must handle invocation at runtime as well. The java.beans.Beans.isDesignTime()
* method can be used to determine if the resolver is being consulted at design-time or runtime.
*/
public abstract class ELResolver {
/**
* The attribute name of the named attribute in the FeatureDescriptor that specifies whether the
* variable or property can be resolved at runtime.
*/
public static final String RESOLVABLE_AT_DESIGN_TIME = "resolvableAtDesignTime";
/**
* The attribute name of the named attribute in the FeatureDescriptor that specifies the runtime
* type of the variable or property.
*/
public static final String TYPE = "type";
/**
* Returns the most general type that this resolver accepts for the property argument, given a
* base object. One use for this method is to assist tools in auto-completion. This assists
* tools in auto-completion and also provides a way to express that the resolver accepts a
* primitive value, such as an integer index into an array. For example, the
* {@link ArrayELResolver} will accept any int as a property, so the return value would be
* Integer.class.
*
* @param context
* The context of this evaluation.
* @param base
* The base object to return the most general property type for, or null to enumerate
* the set of top-level variables that this resolver can evaluate.
* @return null if this ELResolver does not know how to handle the given base object; otherwise
* Object.class if any type of property is accepted; otherwise the most general property
* type accepted for the given base.
*/
public abstract Class getCommonPropertyType(ELContext context, Object base);
/**
* Returns information about the set of variables or properties that can be resolved for the
* given base object. One use for this method is to assist tools in auto-completion. If the base
* parameter is null, the resolver must enumerate the list of top-level variables it can
* resolve. The Iterator returned must contain zero or more instances of
* java.beans.FeatureDescriptor, in no guaranteed order. In the case of primitive types such as
* int, the value null must be returned. This is to prevent the useless iteration through all
* possible primitive values. A return value of null indicates that this resolver does not
* handle the given base object or that the results are too complex to represent with this
* method and the {@link #getCommonPropertyType(ELContext, Object)} method should be used
* instead. Each FeatureDescriptor will contain information about a single variable or property.
* In addition to the standard properties, the FeatureDescriptor must have two named attributes
* (as set by the setValue method):
*
* - {@link #TYPE} - The value of this named attribute must be an instance of java.lang.Class
* and specify the runtime type of the variable or property.
* - {@link #RESOLVABLE_AT_DESIGN_TIME} - The value of this named attribute must be an
* instance of java.lang.Boolean and indicates whether it is safe to attempt to resolve this
* property at designtime. For instance, it may be unsafe to attempt a resolution at design time
* if the ELResolver needs access to a resource that is only available at runtime and no
* acceptable simulated value can be provided.
*
* The caller should be aware that the Iterator returned might iterate through a very large or
* even infinitely large set of properties. Care should be taken by the caller to not get stuck
* in an infinite loop. This is a "best-effort" list. Not all ELResolvers will return completely
* accurate results, but all must be callable at both design-time and runtime (i.e. whether or
* not Beans.isDesignTime() returns true), without causing errors. The propertyResolved property
* of the ELContext is not relevant to this method. The results of all ELResolvers are
* concatenated in the case of composite resolvers.
*
* @param context
* The context of this evaluation.
* @param base
* The base object whose set of valid properties is to be enumerated, or null to
* enumerate the set of top-level variables that this resolver can evaluate.
* @return An Iterator containing zero or more (possibly infinitely more) FeatureDescriptor
* objects, or null if this resolver does not handle the given base object or that the
* results are too complex to represent with this method
*/
public abstract Iterator getFeatureDescriptors(ELContext context, Object base);
/**
* For a given base and property, attempts to identify the most general type that is acceptable
* for an object to be passed as the value parameter in a future call to the
* {@link #setValue(ELContext, Object, Object, Object)} method. If this resolver handles the
* given (base, property) pair, the propertyResolved property of the ELContext object must be
* set to true by the resolver, before returning. If this property is not true after this method
* is called, the caller should ignore the return value. This is not always the same as
* getValue().getClass(). For example, in the case of an {@link ArrayELResolver}, the getType
* method will return the element type of the array, which might be a superclass of the type of
* the actual element that is currently in the specified array element.
*
* @param context
* The context of this evaluation.
* @param base
* The base object whose property value is to be analyzed, or null to analyze a
* top-level variable.
* @param property
* The property or variable to return the acceptable type for.
* @return If the propertyResolved property of ELContext was set to true, then the most general
* acceptable type; otherwise undefined.
* @throws java.lang.NullPointerException
* if context is null
* @throws PropertyNotFoundException
* if the given (base, property) pair is handled by this ELResolver but the
* specified variable or property does not exist or is not readable.
* @throws ELException
* if an exception was thrown while performing the property or variable resolution.
* The thrown exception must be included as the cause property of this exception, if
* available.
*/
public abstract Class getType(ELContext context, Object base, Object property);
/**
* Attempts to resolve the given property object on the given base object. If this resolver
* handles the given (base, property) pair, the propertyResolved property of the ELContext
* object must be set to true by the resolver, before returning. If this property is not true
* after this method is called, the caller should ignore the return value.
*
* @param context
* The context of this evaluation.
* @param base
* The base object whose property value is to be returned, or null to resolve a
* top-level variable.
* @param property
* The property or variable to be resolved.
* @return If the propertyResolved property of ELContext was set to true, then the result of the
* variable or property resolution; otherwise undefined.
* @throws NullPointerException
* if context is null
* @throws PropertyNotFoundException
* if the given (base, property) pair is handled by this ELResolver but the
* specified variable or property does not exist or is not readable.
* @throws ELException
* if an exception was thrown while performing the property or variable resolution.
* The thrown exception must be included as the cause property of this exception, if
* available.
*/
public abstract Object getValue(ELContext context, Object base, Object property);
/**
* For a given base and property, attempts to determine whether a call to
* {@link #setValue(ELContext, Object, Object, Object)} will always fail. If this resolver
* handles the given (base, property) pair, the propertyResolved property of the ELContext
* object must be set to true by the resolver, before returning. If this property is not true
* after this method is called, the caller should ignore the return value.
*
* @param context
* The context of this evaluation.
* @param base
* The base object whose property value is to be analyzed, or null to analyze a
* top-level variable.
* @param property
* The property or variable to return the read-only status for.
* @return If the propertyResolved property of ELContext was set to true, then true if the
* property is read-only or false if not; otherwise undefined.
* @throws NullPointerException
* if context is null
* @throws PropertyNotFoundException
* if the given (base, property) pair is handled by this ELResolver but the
* specified variable or property does not exist.
* @throws ELException
* if an exception was thrown while performing the property or variable resolution.
* The thrown exception must be included as the cause property of this exception, if
* available.
*/
public abstract boolean isReadOnly(ELContext context, Object base, Object property);
/**
* Attempts to set the value of the given property object on the given base object. If this
* resolver handles the given (base, property) pair, the propertyResolved property of the
* ELContext object must be set to true by the resolver, before returning. If this property is
* not true after this method is called, the caller can safely assume no value has been set.
*
* @param context
* The context of this evaluation.
* @param base
* The base object whose property value is to be set, or null to set a top-level
* variable.
* @param property
* The property or variable to be set.
* @param value
* The value to set the property or variable to.
* @throws NullPointerException
* if context is null
* @throws PropertyNotFoundException
* if the given (base, property) pair is handled by this ELResolver but the
* specified variable or property does not exist.
* @throws PropertyNotWritableException
* if the given (base, property) pair is handled by this ELResolver but the
* specified variable or property is not writable.
* @throws ELException
* if an exception was thrown while attempting to set the property or variable. The
* thrown exception must be included as the cause property of this exception, if
* available.
*/
public abstract void setValue(ELContext context, Object base, Object property, Object value);
/**
* Attempts to resolve and invoke the given method on the given base
* object.
*
*
* If this resolver handles the given (base, method) pair, the propertyResolved
* property of the ELContext object must be set to true by the
* resolver, before returning. If this property is not true after this method is
* called, the caller should ignore the return value.
*
*
*
* A default implementation is provided that returns null so that existing classes that extend
* ELResolver can continue to function.
*
*
* @param context
* The context of this evaluation.
* @param base
* The bean on which to invoke the method
* @param method
* The simple name of the method to invoke. Will be coerced to a String.
* @param paramTypes
* An array of Class objects identifying the method's formal parameter types, in
* declared order. Use an empty array if the method has no parameters. Can be
* null, in which case the method's formal parameter types are assumed
* to be unknown.
* @param params
* The parameters to pass to the method, or null if no parameters.
* @return The result of the method invocation (null if the method has a
* void return type).
* @throws MethodNotFoundException
* if no suitable method can be found.
* @throws ELException
* if an exception was thrown while performing (base, method) resolution. The thrown
* exception must be included as the cause property of this exception, if available.
* If the exception thrown is an InvocationTargetException, extract its
* cause and pass it to the ELException constructor.
* @since 2.2
*/
public Object invoke(ELContext context, Object base, Object method, Class[] paramTypes, Object[] params) {
return null;
}
}