Object Pool Design Pattern
2013-11-26 15:53
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Intent
Object pooling can offer a significant performance boost; it is most effective in situations where the cost of initializing a class instance is high, the rate of instantiation of a class is high, and the number of instantiations in use at any one time is low.
Problem
Object pools (otherwise known as resource pools) are used to manage the object caching. A client with access to a Object pool can avoid creating a new Objects by simply asking the pool for one that has already been instantiated instead. Generally the pool willbe a growing pool, i.e. the pool itself will create new objects if the pool is empty, or we can have a pool, which restricts the number of objects created.
It is desirable to keep all Reusable objects that are not currently in use in the same object pool so that they can be managed by one coherent policy. To achieve this, the Reusable Pool class is designed to be a singleton class.
Discussion
The Object Pool lets others "check out" objects from its pool, when those objects are no longer needed by their processes, they are returned to the pool in order to be reused.However, we don't want a process to have to wait for a particular object to be released, so the Object Pool also instantiates new objects as they are required, but must also implement a facility to clean up unused objects periodically.
Structure
The general idea for the Connection Pool pattern is that if instances of a class can be reused, you avoid creating instances of the class by reusing them.Reusable- Instances of classes
in this role collaborate with other objects for a limited amount of time, then they are no longer needed for that collaboration.
Client- Instances of classes
in this role use Reusable objects.
ReusablePool- Instances of
classes in this role manage Reusable objects for use by Client objects.
Usually, it is desirable to keep all
Reusableobjects
that are not currently in use in the same object pool so that they can be managed by one coherent policy. To achieve this, the
ReusablePool
class is designed to be a singleton class. Its constructor(s) are private, which forces other classes to call its getInstance method to get the one instance of the
ReusablePoolclass.
A Client object calls a
ReusablePoolobject's
acquireReusablemethod
when it needs a
Reusableobject. A
ReusablePoolobject
maintains a collection of
Reusableobjects. It uses
the collection of
Reusableobjects to contain a pool
of
Reusableobjects that are not currently in use.
If there are any
Reusableobjects in the pool when
the
acquireReusablemethod is called, it removes
a
Reusableobject from the pool and returns it. If
the pool is empty, then the
acquireReusablemethod
creates a
Reusableobject if it can. If the
acquireReusablemethod
cannot create a new
Reusableobject, then it waits
until a
Reusableobject is returned to the collection.
Client objects pass a
Reusableobject to a
ReusablePoolobject's
releaseReusablemethod
when they are finished with the object. The
releaseReusablemethod
returns a
Reusableobject to the pool of
Reusableobjects
that are not in use.
In many applications of the Object Pool pattern, there are reasons for limiting the total number of
Reusableobjects
that may exist. In such cases, the
ReusablePoolobject
that creates
Reusableobjects is responsible for
not creating more than a specified maximum number of
Reusableobjects.
If
ReusablePoolobjects are responsible for limiting
the number of objects they will create, then the
ReusablePoolclass
will have a method for specifying the maximum number of objects to be created. That method is indicated in the above diagram as setMaxPoolSize.
Example
Do you like bowling? If you do, you probably know that you should change your shoes when you getting the bowling club. Shoe shelf is wonderful example of Object Pool. Once you want to play, you'll get your pair (aquireReusable)
from it. After the game, you'll return shoes back to the shelf (
releaseReusable).
Check list
Create ObjectPoolclass
with private array of
Objects inside
Create
acquareand
releasemethods
in ObjectPool class
Make sure that your ObjectPool is Singleton
Rules of thumb
The Factory Method pattern can be used to encapsulate the creation logic for objects. However, it does not manage them after their creation, the object pool pattern keeps track of the objects it creates.Object Pools are usually implemented as Singletons.
下面是详细实现代码,在代码里并没有对设置数据池的最大最小数量、客户端数进行设置,代码的意图在于通过设计一个简单的数据池来表达资源池的概念:
// ObjectPool Class public abstract class ObjectPool<T> { private long expirationTime; private Hashtable<T, Long> locked, unlocked; public ObjectPool() { expirationTime = 30000; // 30 seconds locked = new Hashtable<T, Long>(); unlocked = new Hashtable<T, Long>(); } protected abstract T create(); public abstract boolean validate(T o); public abstract void expire(T o); public synchronized T checkOut() { long now = System.currentTimeMillis(); T t; if (unlocked.size() > 0) { Enumeration<T> e = unlocked.keys(); while (e.hasMoreElements()) { t = e.nextElement(); if ((now - unlocked.get(t)) > expirationTime) { // object has expired unlocked.remove(t); expire(t); t = null; } else { if (validate(t)) { unlocked.remove(t); locked.put(t, now); return (t); } else { // object failed validation unlocked.remove(t); expire(t); t = null; } } } } // no objects available, create a new one t = create(); locked.put(t, now); return (t); } public synchronized void checkIn(T t) { locked.remove(t); unlocked.put(t, System.currentTimeMillis()); } } //The three remaining methods are abstract //and therefore must be implemented by the subclass public class JDBCConnectionPool extends ObjectPool<Connection> { private String dsn, usr, pwd; public JDBCConnectionPool(String driver, String dsn, String usr, String pwd) { super(); try { Class.forName(driver).newInstance(); } catch (Exception e) { e.printStackTrace(); } this.dsn = dsn; this.usr = usr; this.pwd = pwd; } @Override protected Connection create() { try { return (DriverManager.getConnection(dsn, usr, pwd)); } catch (SQLException e) { e.printStackTrace(); return (null); } } @Override public void expire(Connection o) { try { ((Connection) o).close(); } catch (SQLException e) { e.printStackTrace(); } } @Override public boolean validate(Connection o) { try { return (!((Connection) o).isClosed()); } catch (SQLException e) { e.printStackTrace(); return (false); } } }
JDBCConnectionPool will allow the application to borrow and return database connections:
public class Main { public static void main(String args[]) { // Do something... ... // Create the ConnectionPool: JDBCConnectionPool pool = new JDBCConnectionPool( "org.hsqldb.jdbcDriver", "jdbc:hsqldb://localhost/mydb", "sa", "secret"); // Get a connection: Connection con = pool.checkOut(); // Use the connection ... // Return the connection: pool.checkIn(con); } }
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