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<?xml version="1.0"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements. See the NOTICE file distributed with
this work for additional information regarding copyright ownership.
The ASF licenses this file to You 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.
-->
<document>
<properties>
<title>Examples</title>
<author email="dev@commons.apache.org">Commons Documentation Team</author>
<author email="rwaldhoff@apache.org">Rodney Waldhoff</author>
<revision>$Id: examples.xml 560804 2007-07-29 20:14:22Z bayard $</revision>
</properties>
<body>
<section name="A Simple Pool Client">
<p>
Suppose you're writing a set of <code>java.io.Reader</code> utilities, and would like to
provide a method for dumping the contents of a <code>Reader</code> to a <code>String</code>.
Here's the code for the <code>ReaderUtil</code>, implemented without an <code>ObjectPool</code>:
</p>
<source>
import java.io.Reader;
import java.io.IOException;
public class ReaderUtil {
public ReaderUtil() {
}
/**
* Dumps the contents of the {@link Reader} to a
* String, closing the {@link Reader} when done.
*/
public String readToString(Reader in) throws IOException {
StringBuffer buf = new StringBuffer();
try {
for(int c = in.read(); c != -1; c = in.read()) {
buf.append((char)c);
}
return buf.toString();
} catch(IOException e) {
throw e;
} finally {
try {
in.close();
} catch(Exception e) {
// ignored
}
}
}
}
</source>
<p>
For the sake of this example, let's assume we want to to pool the <code>StringBuffer</code>s
used to buffer the <code>Reader</code>'s contents. (A pool of <code>StringBuffer</code>s
may or may not be useful in practice. We're just using it as a simple example here.)
</p>
<p>
Let's further assume that a complete pool implementation will be provided via
a constructor. (We'll show you how to create such an implementation in just a moment.)
Then to use the pool we simply call <code>borrowObject</code> to obtain the buffer, and
then call <code>returnObject</code> when we're done with it.
Then a <code>ReaderUtil</code> implementation using a pool of <code>StringBuffer</code>s might look
like this:
</p>
<source>
import org.apache.commons.pool.ObjectPool;
import java.io.Reader;
import java.io.IOException;
public class ReaderUtil {
private ObjectPool pool;
public ReaderUtil(ObjectPool pool) {
this.pool = pool;
}
/**
* Dumps the contents of the {@link Reader} to a
* String, closing the {@link Reader} when done.
*/
public String readToString(Reader in) throws IOException {
StringBuffer buf = null;
try {
buf = (StringBuffer)(pool.borrowObject());
for(int c = in.read(); c != -1; c = in.read()) {
buf.append((char)c);
}
return buf.toString();
} catch(IOException e) {
throw e;
} catch(Exception e) {
throw new RuntimeException("Unable to borrow buffer from pool" +
e.toString());
} finally {
try {
in.close();
} catch(Exception e) {
// ignored
}
try {
if(null != buf) {
pool.returnObject(buf);
}
} catch(Exception e) {
// ignored
}
}
}
}
</source>
<p>
Since we've constrained ourselves to the <code>ObjectPool</code> interface, an arbitrary pool
implementation (returning, in our case, <code>StringBuffer</code>s) can be used. When a different
or "better" pool implemenatation comes along, we can simply drop it into our <code>ReaderUtil</code>
without changing a line of code.
</p>
</section>
<section name="A PoolableObjectFactory">
<p>
Recall that Pool provides a simple toolkit for creating object pools. The
<code>PoolableObjectFactory</code> interface is an important part of this toolkit.
<code>PoolableObjectFactory</code> defines lifecycle methods for pooled objects.
We can use it to separate the kinds of objects that are pooled and how they are
created, persisted, or destroyed, from the pooling algorithm itself.
</p>
<p>
Suppose we have an <code>ObjectPool</code> implementation that accepts a
<code>PoolableObjectFactory</code> (for example, any of the implementations in the
<code>org.apache.commons.pool.impl</code> package). Then we need only provide
the factory implemenation in order to pool a new kind of object.
</p>
<p>
Here's a <code>PoolableObjectFactory</code> implementation that creates
<code>StringBuffer</code>s as used above.
</p>
<source>
import org.apache.commons.pool.BasePoolableObjectFactory;
public class StringBufferFactory extends BasePoolableObjectFactory {
// for makeObject we'll simply return a new buffer
public Object makeObject() {
return new StringBuffer();
}
// when an object is returned to the pool,
// we'll clear it out
public void passivateObject(Object obj) {
StringBuffer buf = (StringBuffer)obj;
buf.setLength(0);
}
// for all other methods, the no-op
// implementation in BasePoolableObjectFactory
// will suffice
}
</source>
<p>
We can, for example, use this factory with the <code>StackObjectPool</code> to instantiate our
<code>ReaderUtil</code> as follows:
</p>
<source>new ReaderUtil(new StackObjectPool(new StringBufferFactory()))</source>
</section>
</body>
</document>
|