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