下面是我在读commons-beanutils.jar时的一个类FastHashMap，
大家从上面往下阅读代码，到最后有个红色代码块，有谁能告诉我蓝色代码为什么不能换成“return (map.put(key, value));”?
map.clone()在这里到底有什么作用呢？
/*
* Copyright 2001-2004 The Apache Software Foundation
*
*
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.apache.commons.collections;
import java.util.Collection;
import
java.util.ConcurrentModificationException;
import
java.util.HashMap;
import java.util.Iterator;
import
java.util.Map;
import java.util.Set;
/**
* <p>A customized implementation of
<code>java.util.HashMap</code> designed
* to operate in a
multithreaded environment where the large majority of
* method calls are
read-only, instead of structural changes. When operating
* in "fast" mode,
read calls are non-synchronized and write calls perform the
* following
steps:</p>
* <ul>
* <li>Clone the existing
collection
* <li>Perform the modification on the clone
*
<li>Replace the existing collection with the (modified) clone
*
</ul>
* <p>When first created, objects of this class default to
"slow" mode, where
* all accesses of any type are synchronized but no
cloning takes place. This
* is appropriate for initially populating the
collection, followed by a switch
* to "fast" mode (by calling
<code>setFast(true)</code>) after initialization
* is
complete.</p>
*
* <p><strong>NOTE</strong>: If
you are creating and accessing a
* <code>HashMap</code> only
within a single thread, you should use
*
<code>java.util.HashMap</code> directly (with no synchronization),
for
* maximum performance.</p>
*
*
<p><strong>NOTE</strong>: <i>This class is not
cross-platform.
* Using it may cause unexpected failures on some
architectures.</i>
* It suffers from the same problems as the
double-checked locking idiom.
* In particular, the instruction that clones
the internal collection and the
* instruction that sets the internal
reference to the clone can be executed
* or perceived out-of-order. This
means that any read operation might fail
* unexpectedly, as it may be
reading the state of the internal collection
* before the internal
collection is fully formed.
* For more information on the double-checked
locking idiom, see the
* <a href="http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html">
*
Double-Checked Locking Idiom Is Broken
Declaration</a>.</p>
*
* @since Commons Collections
1.0
* @version $Revision: 1.1 $ $Date: 2004/05/10 19:51:13 $
*
*
@author Craig R. McClanahan
* @author Stephen Colebourne
*/
public
class FastHashMap extends HashMap {
/**
* The underlying map we are managing.
*/
protected HashMap map = null;
/**
* Are we currently operating in "fast" mode?
*/
protected boolean fast = false;
// Constructors
//
----------------------------------------------------------------------
/**
* Construct an empty map.
*/
public
FastHashMap()
/**
* Construct an empty map with the specified capacity.
*
* @param capacity the initial capacity of the empty map
*/
public FastHashMap(int capacity)
/**
* Construct an empty map with the specified capacity and load
factor.
*
* @param capacity the initial capacity of the empty
map
* @param factor the load factor of the new map
*/
public FastHashMap(int capacity, float factor)
/**
* Construct a new map with the same mappings as the specified
map.
*
* @param map the map whose mappings are to be
copied
*/
public FastHashMap(Map map)
// Property access
//
----------------------------------------------------------------------
/**
* Returns true if this map is operating in fast
mode.
*
* @return true if this map is operating in fast
mode
*/
public boolean getFast()
/**
* Sets whether this map is operating in fast mode.
*
* @param fast true if this map should operate in fast mode
*/
public void setFast(boolean fast)
// Map access
//
----------------------------------------------------------------------
//
These methods can forward straight to the wrapped Map in 'fast' mode.
//
(because they are query methods)
/**
* Return the value to which this map maps the specified key.
Returns
* <code>null</code> if the map contains no mapping
for this key, or if
* there is a mapping with a value of
<code>null</code>. Use the
*
<code>containsKey()</code> method to disambiguate these
cases.
*
* @param key the key whose value is to be
returned
* @return the value mapped to that key, or null
*/
public Object get(Object key) {
if (fast)
else {
synchronized (map)
}
}
/**
* Return the number of key-value mappings in this
map.
*
* @return the current size of the map
*/
public int size() {
if (fast)
else {
synchronized (map)
}
}
/**
* Return <code>true</code> if this map contains
no mappings.
*
* @return is the map currently empty
*/
public boolean isEmpty() {
if (fast)
else {
synchronized (map)
}
}
/**
* Return <code>true</code> if this map contains a
mapping for the
* specified key.
*
* @param key the key
to be searched for
* @return true if the map contains the key
*/
public boolean containsKey(Object key) {
if (fast)
else {
synchronized (map)
}
}
/**
* Return <code>true</code> if this map contains
one or more keys mapping
* to the specified value.
*
*
@param value the value to be searched for
* @return true if the map
contains the value
*/
public boolean containsValue(Object value)
{
if (fast)
else {
synchronized (map)
}
}
// Map modification
//
----------------------------------------------------------------------
//
These methods perform special behaviour in 'fast' mode.
// The map is
cloned, updated and then assigned back.
// See the comments at the top as
to why this won't always work.
/**
* Associate the specified value with the specified key in
this map.
* If the map previously contained a mapping for this key, the
old
* value is replaced and returned.
*
* @param key
the key with which the value is to be associated
* @param value the
value to be associated with this key
* @return the value previously
mapped to the key, or null
*/
public Object
put(Object key, Object value) {
if (fast)
{
synchronized (this)
{
HashMap temp = (HashMap)
map.clone();
Object result =
temp.put(key, value);
map =
temp;
return
(result);
}
} else {
synchronized (map) {
return
(map.put(key, value));
}
}
}
。。。。。。接下去的代码不需要贴了
今天仔细想了一下，应该是这样的：
所谓FastHashMap主要是将原来线程不安全的HashMap改造成线程安全的HashMap,其中还分了两个模式：
当在fast模式下，所有的读操作不存在互斥，所有的读操作和写操作不存在互斥，为了保证读完以后map不被修改，用了clone这个策略，也就是内存拷贝，什么叫内存拷贝？假如一个线程A在读，一个线程B在写，那么B操作的map是A操作的map的拷贝，这样不管B做了什么，都不会影响A的操作，这样就保证了不存在并发修。
当不再fast模式下，为了做到线程同步，只能对整个map加锁，这样保证了map的线程安全性，但是整个map的各种操作都将只是单线程的，所以慢了。