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an Open Source BPMN 2.0 Modeler
/*
* Copyright 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 javax.el;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Constructor;
import java.util.Properties;
/**
* 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. 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 {@link #createValueExpression(ELContext, String, Class)} method is used
* to parse expressions that evaluate to values (both l-values and r-values are supported). The
* {@link #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 {@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}").
*
*/
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.
*
*
* @return An instance of ExpressionFactory.
* @throws ELException
* if a factory class cannot be found or instantiation fails.
*/
public static ExpressionFactory newInstance() {
return 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 constructor of the implementation.
* @return An instance of ExpressionFactory.
* @throws ELException
* if a factory class cannot be found or instantiation fails.
*/
public static ExpressionFactory newInstance(Properties properties) {
ClassLoader classLoader;
try {
classLoader = Thread.currentThread().getContextClassLoader();
} catch (SecurityException e) {
classLoader = ExpressionFactory.class.getClassLoader();
}
String className = null;
String serviceId = "META-INF/services/" + ExpressionFactory.class.getName();
InputStream input = classLoader.getResourceAsStream(serviceId);
try {
if (input != null) {
BufferedReader reader = new BufferedReader(new InputStreamReader(input, "UTF-8"));
className = reader.readLine();
reader.close();
}
} catch (IOException e) {
// do nothing
} finally {
if (input != null) {
try {
input.close();
} catch (Exception io) {
// do nothing
} finally {
input = null;
}
}
}
if (className == null || className.trim().length() == 0) {
try {
String home = System.getProperty("java.home");
if (home != null) {
String path = home + File.separator + "lib" + File.separator + "el.properties";
File file = new File(path);
if (file.exists()) {
input = new FileInputStream(file);
Properties props = new Properties();
props.load(input);
className = props.getProperty(ExpressionFactory.class.getName());
}
}
} catch (IOException e) {
// do nothing
} catch (SecurityException e) {
// do nothing
} finally {
if (input != null) {
try {
input.close();
} catch (IOException io) {
// do nothing
} finally {
input = null;
}
}
}
}
if (className == null || className.trim().length() == 0) {
try {
className = System.getProperty(ExpressionFactory.class.getName());
} catch (Exception se) {
// do nothing
}
}
if (className == null || className.trim().length() == 0) {
className = "de.odysseus.el.ExpressionFactoryImpl";
}
return newInstance(properties, className, classLoader);
}
/**
* Create an ExpressionFactory instance.
*
* @param properties
* Properties passed to the constructor of the implementation.
* @return an instance of ExpressionFactory
* @param className
* The name of the ExpressionFactory class.
* @param classLoader
* The class loader to be used to load the class.
* @return An instance of ExpressionFactory.
* @throws ELException
* if the class could not be found or if it is not a subclass of ExpressionFactory
* or if the class could not be instantiated.
*/
private static ExpressionFactory newInstance(Properties properties, String className, ClassLoader classLoader) {
Class clazz = null;
try {
clazz = classLoader.loadClass(className.trim());
if (!ExpressionFactory.class.isAssignableFrom(clazz)) {
throw new ELException("Invalid expression factory class: " + clazz.getName());
}
} catch (ClassNotFoundException e) {
throw new ELException("Could not find expression factory class", e);
}
try {
if (properties != null) {
Constructor constructor = null;
try {
constructor = clazz.getConstructor(Properties.class);
} catch (Exception e) {
// do nothing
}
if (constructor != null) {
return (ExpressionFactory) constructor.newInstance(properties);
}
}
return (ExpressionFactory) clazz.newInstance();
} catch (Exception e) {
throw new ELException("Could not create expression factory instance", e);
}
}
/**
* Coerces an object to a specific type according to the EL type conversion rules. An
* {@link 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.
* @return the coerced object
* @throws ELException
* if an error results from applying the conversion rules.
*/
public abstract Object coerceToType(Object obj, Class targetType);
/**
* 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, unless
* the method is specified with arguments in the EL expression.
* @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);
/**
* 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 ELException
* Thrown if there are syntactical errors in the provided expression.
* @throws NullPointerException
* if paramTypes is null.
*/
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,
* @return a ValueExpression that wraps the given object instance.
*/
public abstract ValueExpression createValueExpression(Object instance, Class expectedType);
}
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