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A Simple Pool Client

Suppose you're writing a set of java.io.Reader utilities, and would like to provide a method for dumping the contents of a Reader to a String. Here's the code for the ReaderUtil, implemented without an ObjectPool:

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 
            } 
        } 
    } 
}

For the sake of this example, let's assume we want to to pool the StringBuffers used to buffer the Reader's contents. (A pool of StringBuffers may or may not be useful in practice. We're just using it as a simple example here.)

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 borrowObject to obtain the buffer, and then call returnObject when we're done with it. Then a ReaderUtil implementation using a pool of StringBuffers might look like this:

import java.io.IOException;
import java.io.Reader;
import org.apache.commons.pool2.ObjectPool;

public class ReaderUtil {
    
    private ObjectPool<StringBuffer> pool;
    
    public ReaderUtil(ObjectPool<StringBuffer> 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 = 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
            }
        }
    }
}

Since we've constrained ourselves to the ObjectPool interface, an arbitrary pool implementation (returning, in our case, StringBuffers) can be used. When a different or "better" pool implementation comes along, we can simply drop it into our ReaderUtil without changing a line of code.

A PooledObjectFactory

The implementations provided in pool2 wrap pooled objects in PooledObject wrappers for internal use by the pool and object factories. The PooledObjectFactory interface defines lifecycle methods for pooled objects. The simplest way to implement a PoolableObjectFactory is to extend BasePooledObjectFactory.

Here's a PooledObjectFactory implementation that creates StringBuffers as used above.

import org.apache.commons.pool2.BasePooledObjectFactory;
import org.apache.commons.pool2.PooledObject;
import org.apache.commons.pool2.impl.DefaultPooledObject;

public class StringBufferFactory
    extends BasePooledObjectFactory<StringBuffer> {

    @Override
    public StringBuffer create() {
        return new StringBuffer();
    }

    /**
     * Use the default PooledObject implementation.
     */
    @Override
    public PooledObject<StringBuffer> wrap(StringBuffer buffer) {
        return new DefaultPooledObject<StringBuffer>(buffer);
    }

    /**
     * When an object is returned to the pool, clear the buffer.
     */
    @Override
    public void passivateObject(PooledObject<StringBuffer> pooledObject) {
        pooledObject.getObject().setLength(0);
    }

    // for all other methods, the no-op implementation
    // in BasePooledObjectFactory will suffice
}

We can, for example, use this factory with the GenericObjectPool to instantiate our ReaderUtil as follows:

ReaderUtil readerUtil = new ReaderUtil(new GenericObjectPool<StringBuffer>(new StringBufferFactory()));