import java.util.ArrayList;
class PoolItem {
boolean inUse = false;
Object item;
PoolItem(Object item) {
this.item = item;
}
}
class ReleasableReference { // Used to build the proxy
private PoolItem reference;
private boolean released = false;
public ReleasableReference(PoolItem reference) {
this.reference = reference;
}
public Object getReference() {
if (released)
throw new RuntimeException("Tried to use reference after it was released");
return reference.item;
}
public void release() {
released = true;
reference.inUse = false;
}
}
class PoolManager {
private ArrayList items = new ArrayList();
public void add(Object item) {
items.add(new PoolItem(item));
}
public static class EmptyPoolItem {
}
public ReleasableReference get() {
for (int i = 0; i < items.size(); i++) {
PoolItem pitem = (PoolItem) items.get(i);
if (pitem.inUse == false) {
pitem.inUse = true;
return new ReleasableReference(pitem);
}
}
return null;
}
}
interface Connection {
Object get();
void set(Object x);
void release();
}
class ConnectionImplementation implements Connection {
public Object get() {
return null;
}
public void set(Object s) {
}
public void release() {
}
}
class ConnectionPool { // A singleton
private static PoolManager pool = new PoolManager();
private ConnectionPool() {
}
public static void addConnections(int number) {
for (int i = 0; i < number; i++)
pool.add(new ConnectionImplementation());
}
public static Connection getConnection() {
ReleasableReference rr = (ReleasableReference) pool.get();
if (rr == null)
return null;
return new ConnectionProxy(rr);
}
}
class ConnectionProxy implements Connection {
private ReleasableReference implementation;
public ConnectionProxy(ReleasableReference rr) {
implementation = rr;
}
public Object get() {
return ((Connection) implementation.getReference()).get();
}
public void set(Object x) {
((Connection) implementation.getReference()).set(x);
}
public void release() {
implementation.release();
}
}
public class ConnectionPoolProxyDemo {
public static void main(String args[]) {
try {
ConnectionPool.addConnections(5);
Connection c = ConnectionPool.getConnection();
c.set(new Object());
c.get();
c.release();
c = ConnectionPool.getConnection();
c.set(new Object());
c.get();
c.release();
c.get();
} catch (Exception e) {
System.out.println(e.getMessage());
}
}
}