最易懂设计模式解析
模板方法设计模式
Mybatis代理设计模式
Mybatis多级代理
1. 认识适配器模式
1.1 模式定义
将某个类的接口转换成客户端期望的另一个接口表示。使得原本由于接口不兼容而不能一起工作的那些类能在一起工作。
1.2 解决何种问题
解决接口与接口实现类之间继承矛盾问题（在编程实现里面：当一个接口实现另一个接口的时候，有义务把接口中的方法都实现）。
1.3 模式特征
- 使用抽象类分离了接口与接口实现。
- 抽象类分摊接口中的方法。
- 使得接口可以随意的选择接口中的方法来实现。
参考servlet规范中的genericServlet
2. 实例讲解
创建一个学生守则接口；
public interface 学生守则接口{
// 不受实现类欢迎的实现方法
public void 考试后家长签字();
//受实现类欢迎的实现方法
public void 每周免费看一次电影();
}
创建一个零食商贩（小学生干爹）
public abstract class 零食小商贩 implements 学生守则接口{
@Override
public void 考试后家长签字(){
System.out.println("家长签字");
}
}
创建一个小学类
public class 小学生 extends 零食小商贩{
@Override
public void 每周去看一次电影（）{
System.out.println("去看电影");
}
}
定义目标类，通过小学生类调用所需要的方法
public class TestMain{
public static void main(String[] args){
学生守则接口 target = new 小学生();
target.考试后家长签字();
target.每周免费看一次电影();
}
}
抽象类实现接口的时候，对接口中的方法可以选择重写也可以选择不重写，不重写的方法由子类来完成。（目的降低接口开发的难度）
3. 适配器模式在servlet中的应用
下面直接看源码，发现GenericServlet是一个抽象类，它实现了Servlet接口，它把servlet一些不重要的方法（比较烦的方法）全部实现了，看下面destroy()、init()等大部分方法都是空实现。其service()是一个抽象方法，即把它处理请求的任务交给子类。子类必须实现该方法。
看下面GenericServlet的描述，简译就是，GenericServlet定义了一个通用的无关协议的Servlet。如果要在web应用中使用Http进行Servlet通信，请扩展HttpServlet。
package javax.servlet;
import java.io.IOException;
import java.util.Enumeration;
import java.util.ResourceBundle;
/**
*
* Defines a generic, protocol-independent
* servlet. To write an HTTP servlet for use on the
* Web, extend {@link javax.servlet.http.HttpServlet} instead.
*
* GenericServlet implements the Servlet
* and ServletConfig interfaces. GenericServlet
* may be directly extended by a servlet, although it's more common to extend
* a protocol-specific subclass such as HttpServlet.
*
*
GenericServlet makes writing servlets
* easier. It provides simple versions of the lifecycle methods
* init and destroy and of the methods
* in the ServletConfig interface. GenericServlet
* also implements the log method, declared in the
* ServletContext interface.
*
*
To write a generic servlet, you need only
* override the abstract service method.
*
*
* @author Various
*/
public abstract class GenericServlet
implements Servlet, ServletConfig, java.io.Serializable
{
private static final String LSTRING_FILE = "javax.servlet.LocalStrings";
private static ResourceBundle lStrings =
ResourceBundle.getBundle(LSTRING_FILE);
private transient ServletConfig config;
/**
*
* Does nothing. All of the servlet initialization
* is done by one of the init methods.
*
*/
public GenericServlet() { }
/**
* Called by the servlet container to indicate to a servlet that the
* servlet is being taken out of service. See {@link Servlet#destroy}.
*
*
*/
public void destroy() {
}
/**
* Returns a String containing the value of the named
* initialization parameter, or null if the parameter does
* not exist. See {@link ServletConfig#getInitParameter}.
*
*
This method is supplied for convenience. It gets the
* value of the named parameter from the servlet's
* ServletConfig object.
*
* @param name a String specifying the name
* of the initialization parameter
*
* @return String a String containing the value
* of the initialization parameter
*
*/
public String getInitParameter(String name) {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameter(name);
}
/**
* Returns the names of the servlet's initialization parameters
* as an Enumeration of String objects,
* or an empty Enumeration if the servlet has no
* initialization parameters. See {@link
* ServletConfig#getInitParameterNames}.
*
*
This method is supplied for convenience. It gets the
* parameter names from the servlet's ServletConfig object.
*
*
* @return Enumeration an enumeration of String
* objects containing the names of
* the servlet's initialization parameters
*/
public Enumeration getInitParameterNames() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameterNames();
}
/**
* Returns this servlet's {@link ServletConfig} object.
*
* @return ServletConfig the ServletConfig object
* that initialized this servlet
*/
public ServletConfig getServletConfig() {
return config;
}
/**
* Returns a reference to the {@link ServletContext} in which this servlet
* is running. See {@link ServletConfig#getServletContext}.
*
* This method is supplied for convenience. It gets the
* context from the servlet's ServletConfig object.
*
*
* @return ServletContext the ServletContext object
* passed to this servlet by the init
* method
*/
public ServletContext getServletContext() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletContext();
}
/**
* Returns information about the servlet, such as
* author, version, and copyright.
* By default, this method returns an empty string. Override this method
* to have it return a meaningful value. See {@link
* Servlet#getServletInfo}.
*
*
* @return String information about this servlet, by default an
* empty string
*/
public String getServletInfo() {
return "";
}
/**
* Called by the servlet container to indicate to a servlet that the
* servlet is being placed into service. See {@link Servlet#init}.
*
*
This implementation stores the {@link ServletConfig}
* object it receives from the servlet container for later use.
* When overriding this form of the method, call
* super.init(config).
*
* @param config the ServletConfig object
* that contains configutation
* information for this servlet
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's normal
* operation
*
* @see UnavailableException
*/
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
/**
* A convenience method which can be overridden so that there's no need
* to call super.init(config).
*
*
Instead of overriding {@link #init(ServletConfig)}, simply override
* this method and it will be called by
* GenericServlet.init(ServletConfig config).
* The ServletConfig object can still be retrieved via {@link
* #getServletConfig}.
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's
* normal operation
*/
public void init() throws ServletException {
}
/**
* Writes the specified message to a servlet log file, prepended by the
* servlet's name. See {@link ServletContext#log(String)}.
*
* @param msg a String specifying
* the message to be written to the log file
*/
public void log(String msg) {
getServletContext().log(getServletName() + ": "+ msg);
}
/**
* Writes an explanatory message and a stack trace
* for a given Throwable exception
* to the servlet log file, prepended by the servlet's name.
* See {@link ServletContext#log(String, Throwable)}.
*
*
* @param message a String that describes
* the error or exception
*
* @param t the java.lang.Throwable error
* or exception
*/
public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
/**
* Called by the servlet container to allow the servlet to respond to
* a request. See {@link Servlet#service}.
*
*
This method is declared abstract so subclasses, such as
* HttpServlet, must override it.
*
* @param req the ServletRequest object
* that contains the client's request
*
* @param res the ServletResponse object
* that will contain the servlet's response
*
* @exception ServletException if an exception occurs that
* interferes with the servlet's
* normal operation occurred
*
* @exception IOException if an input or output
* exception occurs
*/
public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
/**
* Returns the name of this servlet instance.
* See {@link ServletConfig#getServletName}.
*
* @return the name of this servlet instance
*/
public String getServletName() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletName();
}
}
模仿上面写一个
建一个servlet类叫MyGenericServlet，实现servlet接口，实现里面的方法，留service()方法由他的子类来实现。
public abstract class MyGenericServlet implements Servlet{
@Override
public void destroy() {
// TODO Auto-generated method stub
}
@Override
public ServletConfig getServletConfig() {
// TODO Auto-generated method stub
return null;
}
@Override
public String getServletInfo() {
return null;
}
@Override
public void init(ServletConfig arg0) throws ServletException {
}
}
创建一个OneServlet类
/**
* Servlet implementation class OneServlet
*/
public class OneServlet extends MyGenericServlet {
@Override
public void service(ServletRequest arg0, ServletResponse arg1) throws ServletException, IOException {
System.out.println("oneServlet is run..");
}
}
可以测试localhost:8080/.../...