Builder Design Pattern

Intent

Seperate the construction of a complex object from it representation so that the same construction process can create different representations.

Applicability

Use builder pattern when

The algoritm for creating a complex object should be independent of the parts that make up the object and how they are assembled.
The construction process must allow different representations for the object that is constructed.
More general usage is when your class constructor requires too many parameters.(Recommended by Effective Java by Josh Bloch)

UML diagram

Participants

Builder
- Defines an abstract interface for creating parts of the product.
Concrete Builder
- Defines interface for retrieving the product.
- Builds the products internal representation and defines the process by which it is assembled.
Director
- Constructs the product using the builder interface
Product
- Represents the complex object under construction.

Coonsequences

Lets you vary a products internal representation
Isolate code for construction and representation
It gives finer control over the construction process

Implementation

Here are the implementation issues to consider

Builder interface must be general enough to allow all kinds of concrete builders for construction of products.
The Builder methods are defined as empty methods instead of declaring them as abstract(pure virtual in C++). This is to let the client override only the operations they are interested in.

Example Code

Below is the code for implementing Connection Pool. Here we used builder to avoid contructor with more parameters as specified in Usege #3. In this case there is only one Builder so we have created inner class inside the ConnectionPool class. In case of other usages there can exist more than one representation of the product and each representation is constructed by different Builder. And we construct Director using the appropriate Builder object.

Connection.java

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package com.practice.connectionpool.pool;

public class Connection {
	public static Integer count = 0;
	private String userName;
	private String password;
	private String connectionUrl;
	private Integer id;
	
	public Connection(String userName, String password, String connectionUrl){
		this.userName = userName;
		this.password = password;
		this.connectionUrl = connectionUrl;
		count++;
		this.id = count;		
	}
	public static Integer getCount() {
		return count;
	}
	public static void setCount(Integer count) {
		Connection.count = count;
	}
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getUserName() {
		return userName;
	}
	public void setUserName(String userName) {
		this.userName = userName;
	}
	public String getPassword() {
		return password;
	}
	public void setPassword(String password) {
		this.password = password;
	}
	public String getConnectionUrl() {
		return connectionUrl;
	}
	public void setConnectionUrl(String connectionUrl) {
		this.connectionUrl = connectionUrl;
	}	
}

IConnectionPool.java

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package com.practice.connectionpool.pool;

public interface IConnectionPool {
	public Connection getConnection();
	public void releaseConnection(Connection con);
}

ConnectionPool.java

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package com.practice.connectionpool.pool;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ConnectionPool implements IConnectionPool {

	private String userName;
	private String password;
	private String connectionUrl;
	private Integer maxSize;
	private Integer minSize;
	
	List<Connection> availableConnections;
	Map<Connection, Connection> usedConnections;
	Lock lock;
	
	public ConnectionPool() {
		lock = new ReentrantLock();
		
		availableConnections = new ArrayList<>();
		usedConnections = new HashMap<>();
	}
	
	protected void init() {
		/* Create predefined number of connections */
		for(int i=0; i < minSize; i++) {
			availableConnections.add(new Connection(userName,password,connectionUrl));
		}		
	}
	
	public String getUserName() {
		return userName;
	}

	public void setUserName(String userName) {
		lock.lock();
		this.userName = userName;
		lock.unlock();
	}

	public String getPassword() {
		return password;
	}

	public void setPassword(String password) {
		lock.lock();
		this.password = password;
		lock.unlock();
	}

	public String getConnectionUrl() {
		return connectionUrl;
	}

	public void setConnectionUrl(String connectionUrl) {
		lock.lock();
		this.connectionUrl = connectionUrl;
		lock.unlock();
	}

	public List<Connection> getAvailableConnections() {
		return availableConnections;
	}

	public void setAvailableConnections(List<Connection> availableConnections) {
		lock.lock();
		this.availableConnections = availableConnections;
		lock.unlock();
	}

	public Integer getMaxSize() {
		return maxSize;
	}

	public void setMaxSize(Integer maxSize) {
		lock.lock();
		this.maxSize = maxSize;
		lock.unlock();
	}

	public Integer getMinSize() {
		return minSize;
	}

	public void setMinSize(Integer minSize) {
		lock.lock();
		this.minSize = minSize;
		lock.unlock();
	}

	public Map<Connection, Connection> getUsedConnections() {
		return usedConnections;
	}

	public void setUsedConnections(Map<Connection, Connection> usedConnections) {
		lock.lock();
		this.usedConnections = usedConnections;
		lock.unlock();
	}

	@Override
	public Connection getConnection() {
		
		Connection con = null;
		//We have already used maximum number of connections
		if(getUsedConnections().size() >= getMaxSize())
			return null;
		
		lock.lock();
		if(getAvailableConnections().size() !=0 ) {
			con = getAvailableConnections().remove(0);
			getUsedConnections().put(con,con);
		}
		else {
			con = new Connection(userName,password,connectionUrl);
			getUsedConnections().put(con,con);
		}
		lock.unlock();
		
		return con;
	}

	@Override
	public void releaseConnection(Connection con) {
		lock.lock();
		getAvailableConnections().add(getUsedConnections().remove(con));
		lock.unlock();
		System.out.println("Released connection : " + con.getId());
	}
	
	public static class Builder{
		private String userName;
		private String password;
		private String connectionUrl;
		private Integer maxSize;
		private Integer minSize;
		public String getUserName() {
			return userName;
		}
		public Builder setUserName(String userName) {
			this.userName = userName;
			return this;
		}
		public String getPassword() {
			return password;
		}
		public Builder setPassword(String password) {
			this.password = password;
			return this;
		}
		public String getConnectionUrl() {
			return connectionUrl;
		}
		public Builder setConnectionUrl(String connectionUrl) {
			this.connectionUrl = connectionUrl;
			return this;
		}
		public Integer getMaxSize() {
			return maxSize;
		}
		public Builder setMaxSize(Integer maxSize) {
			this.maxSize = maxSize;
			return this;
		}
		public Integer getMinSize() {
			return minSize;
		}
		public Builder setMinSize(Integer minSize) {
			this.minSize = minSize;
			return this;
		}
		
		public ConnectionPool build() {
			ConnectionPool pool = new ConnectionPool();
			pool.setUserName(userName);
			pool.setPassword(password);
			pool.setConnectionUrl(connectionUrl);
			if(maxSize != 0)
				pool.setMaxSize(maxSize);
			else
				pool.setMaxSize(5);
			if(minSize != 0)
				pool.setMinSize(minSize);
			else
				pool.setMinSize(3);
			pool.init();
			return pool;
		}
	}
}

ConnectionManager.java

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package com.practice.connectionpool.manager;

import com.practice.connectionpool.pool.Connection;
import com.practice.connectionpool.pool.ConnectionPool;
import com.practice.connectionpool.pool.IConnectionPool;

public class ConnectionManager {

	private static ConnectionManager manager = null;
	IConnectionPool pool = null;
	
	private ConnectionManager(){
		init();
	}	
	public static ConnectionManager getConnectionManager() {
		if(manager == null) {
			synchronized(ConnectionManager.class) {
				if(manager == null)
					manager = new ConnectionManager();
			}
		}		
		return manager;
	}
	
	public void init(){
		
		pool = new ConnectionPool.Builder()
				.setUserName("dsama")
				.setPassword("password")
				.setConnectionUrl("")
				.setMaxSize(10)
				.setMinSize(2)
				.build();
	}	
	public Connection getConnetion() {
		return pool.getConnection();
	}	
	public void releaseConnection(Connection con) {
		pool.releaseConnection(con);
	}
}

ConnectionPoolTest.java

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package com.practice.connectionpool.test;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

import com.practice.connectionpool.manager.ConnectionManager;
import com.practice.connectionpool.pool.Connection;

public class ConnectionPoolTest implements Runnable {

	public void run(){
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		ConnectionManager manager = ConnectionManager.getConnectionManager();
		Connection connection = manager.getConnetion();
		System.out.println("Thread id : " + Thread.currentThread().getId() + "Connection id : " + connection.getId());

		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			
			e.printStackTrace();
		}
		manager.releaseConnection(connection);	
	}
	
	public static void main(String[] args) throws Exception {		
		
		ExecutorService executor = Executors.newFixedThreadPool(6);
		for(int i=0;i < 20; i++) {
			executor.submit(new ConnectionPoolTest());
		}		
		executor.awaitTermination(1, TimeUnit.DAYS);
		executor.shutdown();		
	}
}

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