package com.cuishen.callback;public class Context implements A.Callback {	private A a;		public void begin() {		System.out.println("begin ...");	}	public void end() {		System.out.println("end ...");	}		public Context() {		this.a = new A(this);	}		public void doSomething() {		this.a.doIt();	}		public static void main(String args[]) {		new Context().doSomething();	}}package com.cuishen.callback;public class A {		private final Callback callback;		public static interface Callback {		public void begin();		public void end();	}	public A(Callback callback) {		this.callback = callback;	}	public void doIt() {		callback.begin();		System.out.println("do something ...");		callback.end();	}}

上面的代码模型其原型是出自hibernate里的org.hibernate.jdbc.JDBCContext 和 org.hibernate.jdbc.ConnectionManager两个类，从上面的模型不难看出：Context类实现了A类的Callback接口，在Context类的构造器里将自己注入了A类，在Context类里调用A类的doIt()方法，这时就是：you call me；在doIt()方法体里调用了Context类的begin()和end()方法，这时就是：i call back。Context类 和 A类相互依赖，互相调用 

package com.cuishen.callback;import java.sql.Connection;import java.sql.DriverManager;import java.sql.SQLException;public class Dao {	private interface Callback {		Object doIt(Connection conn) throws SQLException;	}	private Object execute(Callback callback) throws SQLException {		Connection conn = openConnection(); // 开启数据库连接		try { return callback.doIt(conn); } // 执行具体操作并返回操作结果		finally { closeConnection(conn); } // 关闭数据库连接	}		public Object sqlQuery(final String sql) throws SQLException {		return execute(			new Callback() {				public Object doIt(Connection conn) throws SQLException {					return conn.createStatement().executeQuery(sql);				}			}		);	}		public Connection openConnection() throws SQLException {		return DriverManager.getConnection("", null);	}	public void closeConnection(Connection conn) throws SQLException {		if(conn != null && !conn.isClosed()) {			conn.close();		}	}}