Decorator Pattern

装饰模式是以对客户端透明的方式扩展对象的功能。是继承的替代方案。


UML图

参照夏昕的hibernate书中的连接池例子，来了解装饰模式如何应用。

先新建一个连接池接口。
ConnectionPool.java

package com.javapatterns.decorator.jdbc;

import java.sql.Connection;

public interface ConnectionPool {
	public Connection getConnection();
	public void releaseConncetion(Connection conn) ;
}

新建一个连接池
DBConnectionPool.java

package com.javapatterns.decorator.jdbc;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;
import java.util.Vector;

public class DBConnectionPool implements ConnectionPool{

	private Vector pool;
	private Map stmtPool = new HashMap();
	private static final int POOL_MAX_SIZE = 30;

	public synchronized Connection getConnection() {
		if(pool == null)
			pool = new Vector();
		Connection conn;
		if (pool.isEmpty())
			conn = createConnection();
		else {
			int last_idx = pool.size() - 1;
			conn = (Connection)pool.get(last_idx);
			pool.remove(last_idx);
		}		
		return conn;
	}

	private Connection createConnection(){
		Connection conn = null;
		try {
			Class.forName("org.gjt.mm.mysql.Driver");
			conn = DriverManager.getConnection("dbc:mysql://localhost:3306/test","root","root");
			return conn;
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
		} catch (SQLException e) {
			e.printStackTrace();
		}
		return null;
	}

	public void releaseConncetion(Connection conn) {
		if (pool.size() > POOL_MAX_SIZE) {
			try {
				conn.close();
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
		else {
			pool.add(conn);
		}
	}
}

DBConnectionPool是一个简陋的连接池，它有一个问题，当客户端调用Connection.close()关闭连接,而不是调用DBConnectionPool.releaseConncetion(Connection conn)返还Connection.这样会导致连接池彻底失效。

新建一个Dectorator

package com.javapatterns.decorator.jdbc;

import java.sql.CallableStatement;
import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.sql.SQLWarning;
import java.sql.Savepoint;
import java.sql.Statement;
import java.util.Map;

public class ConnectionDectorator implements Connection {
	Connection conn;
	
	public ConnectionDectorator(Connection conn) {
		this.conn = conn;
	}
	
	public void clearWarnings() throws SQLException {
		this.conn.clearWarnings();
	}

	public void close() throws SQLException {
		this.conn.close();
	}

	public void commit() throws SQLException {
		this.conn.commit();
	}

	public Statement createStatement() throws SQLException {
		return this.conn.createStatement();
	}

	public Statement createStatement(int resultSetType, int resultSetConcurrency)
			throws SQLException {
		return this.conn.createStatement(resultSetType, resultSetConcurrency);
	}

	public Statement createStatement(int resultSetType,
			int resultSetConcurrency, int resultSetHoldability)
			throws SQLException {
		return this.conn.createStatement(resultSetType, resultSetConcurrency, resultSetHoldability);
	}

	public boolean getAutoCommit() throws SQLException {
		return this.conn.getAutoCommit();
	}

	public String getCatalog() throws SQLException {
		return this.conn.getCatalog();
	}

	public int getHoldability() throws SQLException {
		return this.conn.getHoldability();
	}

	public DatabaseMetaData getMetaData() throws SQLException {
		return this.conn.getMetaData();
	}

	public int getTransactionIsolation() throws SQLException {
		return this.conn.getTransactionIsolation();
	}

	public Map getTypeMap() throws SQLException {
		return this.conn.getTypeMap();
	}

	public SQLWarning getWarnings() throws SQLException {
		return this.conn.getWarnings();
	}

	public boolean isClosed() throws SQLException {
		return this.conn.isClosed();
	}

	public boolean isReadOnly() throws SQLException {
		return this.conn.isReadOnly();
	}

	public String nativeSQL(String sql) throws SQLException {
		return this.conn.nativeSQL(sql);
	}

	public CallableStatement prepareCall(String sql) throws SQLException {
		return this.conn.prepareCall(sql);
	}

	public CallableStatement prepareCall(String sql, int resultSetType,
			int resultSetConcurrency) throws SQLException {
		return this.conn.prepareCall(sql, resultSetType, resultSetConcurrency);
	}

	public CallableStatement prepareCall(String sql, int resultSetType,
			int resultSetConcurrency, int resultSetHoldability)
			throws SQLException {
		return this.conn.prepareCall(sql, resultSetType, resultSetConcurrency, resultSetHoldability);
	}

	public PreparedStatement prepareStatement(String sql) throws SQLException {
		return this.conn.prepareStatement(sql);
	}

	public PreparedStatement prepareStatement(String sql, int autoGeneratedKeys)
			throws SQLException {
		return this.conn.prepareStatement(sql, autoGeneratedKeys);
	}

	public PreparedStatement prepareStatement(String sql, int[] columnIndexes)
			throws SQLException {
		return this.conn.prepareStatement(sql, columnIndexes);
	}

	public PreparedStatement prepareStatement(String sql, String[] columnNames)
			throws SQLException {
		return this.conn.prepareStatement(sql, columnNames);
	}

	public PreparedStatement prepareStatement(String sql, int resultSetType,
			int resultSetConcurrency) throws SQLException {
		return this.conn.prepareStatement(sql, resultSetType, resultSetConcurrency);
	}

	public PreparedStatement prepareStatement(String sql, int resultSetType,
			int resultSetConcurrency, int resultSetHoldability)
			throws SQLException {
		return this.conn.prepareStatement(sql, resultSetType, resultSetConcurrency);
	}

	public void releaseSavepoint(Savepoint savepoint) throws SQLException {
		this.conn.releaseSavepoint(savepoint);
	}

	public void rollback() throws SQLException {
		this.conn.rollback();

	}

	public void rollback(Savepoint savepoint) throws SQLException {
		this.conn.rollback(savepoint);
	}

	public void setAutoCommit(boolean autoCommit) throws SQLException {
		this.conn.setAutoCommit(autoCommit);
	}

	public void setCatalog(String catalog) throws SQLException {
		this.conn.setCatalog(catalog);
	}

	public void setHoldability(int holdability) throws SQLException {
		this.conn.setHoldability(holdability);
	}

	public void setReadOnly(boolean readOnly) throws SQLException {
		this.conn.setReadOnly(readOnly);
	}

	public Savepoint setSavepoint() throws SQLException {
		return this.conn.setSavepoint();
	}

	public Savepoint setSavepoint(String name) throws SQLException {
		return this.conn.setSavepoint(name);
	}

	public void setTransactionIsolation(int level) throws SQLException {
		this.conn.setTransactionIsolation(level);
	}

	public void setTypeMap(Map arg0) throws SQLException {
		this.conn.setTypeMap(arg0);
	}

}

新建一个Concrete Decorator 

package com.javapatterns.decorator.jdbc;

import java.sql.Connection;
import java.sql.SQLException;

public class PooledConnection extends ConnectionDectorator {
	private ConnectionPool pool;

	public PooledConnection(ConnectionPool pool, Connection conn) {
		super(conn);
		this.pool = pool;
	}

	@Override
	public void close() throws SQLException {
		pool.releaseConncetion(conn);
	}
}

修改DBConnectionPool

package com.javapatterns.decorator.jdbc;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.HashMap;
import java.util.Map;
import java.util.Vector;

public class DBConnectionPool implements ConnectionPool{

	private Vector pool;
	private Map stmtPool = new HashMap();
	private static final int POOL_MAX_SIZE = 30;

	public synchronized Connection getConnection() {
		if(pool == null)
			pool = new Vector();
		Connection conn;
		if (pool.isEmpty())
			conn = createConnection();
		else {
			int last_idx = pool.size() - 1;
			conn = (Connection)pool.get(last_idx);
			pool.remove(last_idx);
		}
		//decorator implementation
		return new PooledConnection(this, conn);
	}
...
}

当有多个Concrete Decorator类动态组合时，更能体现装饰模式的威力。当然，这里用动态代理代码会更简洁一些。