ajava.beans.Expression Maven / Gradle / Ivy
Show all versions of a-javabeans8 Show documentation
/*
* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package ajava.beans;
/**
* An Expression object represents a primitive expression
* in which a single method is applied to a target and a set of
* arguments to return a result - as in "a.getFoo()".
*
* In addition to the properties of the super class, the
* Expression object provides a value which
* is the object returned when this expression is evaluated.
* The return value is typically not provided by the caller and
* is instead computed by dynamically finding the method and invoking
* it when the first call to getValue is made.
*
* @see #getValue
* @see #setValue
*
* @since 1.4
*
* @author Philip Milne
*/
public class Expression extends Statement {
private static Object unbound = new Object();
private Object value = unbound;
/**
* Creates a new {@link Expression} object
* for the specified target object to invoke the method
* specified by the name and by the array of arguments.
*
* The {@code target} and the {@code methodName} values should not be {@code null}.
* Otherwise an attempt to execute this {@code Expression}
* will result in a {@code NullPointerException}.
* If the {@code arguments} value is {@code null},
* an empty array is used as the value of the {@code arguments} property.
*
* @param target the target object of this expression
* @param methodName the name of the method to invoke on the specified target
* @param arguments the array of arguments to invoke the specified method
*
* @see #getValue
*/
@ConstructorProperties({"target", "methodName", "arguments"})
public Expression(Object target, String methodName, Object[] arguments) {
super(target, methodName, arguments);
}
/**
* Creates a new {@link Expression} object with the specified value
* for the specified target object to invoke the method
* specified by the name and by the array of arguments.
* The {@code value} value is used as the value of the {@code value} property,
* so the {@link #getValue} method will return it
* without executing this {@code Expression}.
*
* The {@code target} and the {@code methodName} values should not be {@code null}.
* Otherwise an attempt to execute this {@code Expression}
* will result in a {@code NullPointerException}.
* If the {@code arguments} value is {@code null},
* an empty array is used as the value of the {@code arguments} property.
*
* @param value the value of this expression
* @param target the target object of this expression
* @param methodName the name of the method to invoke on the specified target
* @param arguments the array of arguments to invoke the specified method
*
* @see #setValue
*/
public Expression(Object value, Object target, String methodName, Object[] arguments) {
this(target, methodName, arguments);
setValue(value);
}
/**
* {@inheritDoc}
*
* If the invoked method completes normally,
* the value it returns is copied in the {@code value} property.
* Note that the {@code value} property is set to {@code null},
* if the return type of the underlying method is {@code void}.
*
* @throws NullPointerException if the value of the {@code target} or
* {@code methodName} property is {@code null}
* @throws NoSuchMethodException if a matching method is not found
* @throws SecurityException if a security manager exists and
* it denies the method invocation
* @throws Exception that is thrown by the invoked method
*
* @see java.lang.reflect.Method
* @since 1.7
*/
@Override
public void execute() throws Exception {
setValue(invoke());
}
/**
* If the value property of this instance is not already set,
* this method dynamically finds the method with the specified
* methodName on this target with these arguments and calls it.
* The result of the method invocation is first copied
* into the value property of this expression and then returned
* as the result of getValue. If the value property
* was already set, either by a call to setValue
* or a previous call to getValue then the value
* property is returned without either looking up or calling the method.
*
* The value property of an Expression is set to
* a unique private (non-null) value by default and
* this value is used as an internal indication that the method
* has not yet been called. A return value of null
* replaces this default value in the same way that any other value
* would, ensuring that expressions are never evaluated more than once.
*
* See the execute method for details on how
* methods are chosen using the dynamic types of the target
* and arguments.
*
* @see Statement#execute
* @see #setValue
*
* @return The result of applying this method to these arguments.
* @throws Exception if the method with the specified methodName
* throws an exception
*/
public Object getValue() throws Exception {
if (value == unbound) {
setValue(invoke());
}
return value;
}
/**
* Sets the value of this expression to value.
* This value will be returned by the getValue method
* without calling the method associated with this
* expression.
*
* @param value The value of this expression.
*
* @see #getValue
*/
public void setValue(Object value) {
this.value = value;
}
/*pp*/ String instanceName(Object instance) {
return instance == unbound ? "" : super.instanceName(instance);
}
/**
* Prints the value of this expression using a Java-style syntax.
*/
public String toString() {
return instanceName(value) + "=" + super.toString();
}
}