Avoid expensive acquisition and release of resources by recycling objects that are no longer in use. Can be considered a generalization of connection pool and thread pool patterns.
Handling of empty pools
Object pools employ one of three strategies to handle a request when there are no spare objects in the pool.
- Fail to provide an object (and return an error to the client).
- Allocate a new object, thus increasing the size of the pool. Pools that do this usually allow you to set the high water mark (the maximum number of objects ever used).
- In a multi-threaded environment, a pool may block the client until another thread returns an object to the pool.
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);
}
}
Computing Codes 