如何学习JDK里的设计模式

前言

最近在看JDK源码，想在毕业前再好好提高一下写代码的能力，JDK是个优秀的源码阅读范本（Spring的源码也不错）。JDK目录下的src.zip里可以直接获得源码，我也push到了我Github的一个repo里。

网上搜了JDK设计模式，coolshell里也有一篇，不过我还是参照了Stackoverflow(原文链接)上的一个“Examples of GoF Design Patterns”的答复，详细列举了JDK里具体对应的类和函数，并结合了wiki上的许多整理好的优秀词条内容。放到这个博客上，也方便自己深入学习，好好体会下设计模式内涵，光看书也会很枯燥，况且纸质的代码看起来也不舒服。

正文

You can find an overview of a lot of design patterns inWikipedia. It also mentions which patterns are mentioned by GoF. I'll sum them up here and try to assign as much as possible pattern implementations found in both the Java SE and Java EE API's.

Creational patterns

Abstract factory^{_{(recognizeable by creational methods returning the factory itself which in turn can be used to create another abstract/interface type)}}

• javax.xml.parsers.DocumentBuilderFactory#newInstance()
• javax.xml.transform.TransformerFactory#newInstance()
• javax.xml.xpath.XPathFactory#newInstance()

Builder^{_{(recognizeable by creational methods returning the instance itself)}}

• java.lang.StringBuilder#append()(unsynchronized)
• java.lang.StringBuffer#append()(synchronized)
• java.nio.ByteBuffer#put()(also onCharBuffer,ShortBuffer,IntBuffer,LongBuffer,FloatBufferandDoubleBuffer)
• javax.swing.GroupLayout.Group#addComponent()
• All implementations ofjava.lang.Appendable

Factory method^{_{(recognizeable by creational methods returning an implementation of an abstract/interface type)}}

• java.util.Calendar#getInstance()
• java.util.ResourceBundle#getBundle()
• java.text.NumberFormat#getInstance()
• java.nio.charset.Charset#forName()
• java.net.URLStreamHandlerFactory#createURLStreamHandler(String)(Returns singleton object per protocol)

Prototype^{_{(recognizeable by creational methods returning adifferentinstance of itself with the same properties)}}

• java.lang.Object#clone()(the class has to implementjava.lang.Cloneable)

Singleton^{_{(recognizeable by creational methods returning thesameinstance (usually of itself) everytime)}}

• java.lang.Runtime#getRuntime()
• java.awt.Desktop#getDesktop()

Structural patterns

Adapter^{_{(recognizeable by creational methods taking an instance ofdifferentabstract/interface type and returning an implementation of own/another abstract/interface type whichdecorates/overridesthe given instance)}}

• java.util.Arrays#asList()
• java.io.InputStreamReader(InputStream)(returns aReader)
• java.io.OutputStreamWriter(OutputStream)(returns aWriter)
• javax.xml.bind.annotation.adapters.XmlAdapter#marshal()and#unmarshal()

Bridge^{_{(recognizeable by creational methods taking an instance ofdifferentabstract/interface type and returning an implementation of own abstract/interface type whichdelegates/usesthe given instance)}}

• None comes to mind yet. A fictive example would benew LinkedHashMap(LinkedHashSet, List)which returns an unmodifiable linked map which doesn't clone the items, butusesthem. Thejava.util.Collections#newSetFromMap()andsingletonXXX()methods however comes close.

Composite^{_{(recognizeable by behavioral methods taking an instance ofsameabstract/interface type into a tree structure)}}

• java.awt.Container#add(Component)(practically all over Swing thus)
• javax.faces.component.UIComponent#getChildren()(practically all over JSF UI thus)

Decorator^{_{(recognizeable by creational methods taking an instance ofsameabstract/interface type which adds additional behaviour)}}

• All subclasses ofjava.io.InputStream,OutputStream,ReaderandWriterhave a constructor taking an instance of same type.
• java.util.Collections, thecheckedXXX(),synchronizedXXX()andunmodifiableXXX()methods.
• javax.servlet.http.HttpServletRequestWrapperandHttpServletResponseWrapper

Facade^{_{(recognizeable by behavioral methods which internally uses instances ofdifferentindependent abstract/interface types)}}

• javax.faces.context.FacesContext, it internally uses among others the abstract/interface typesLifeCycle,ViewHandler,NavigationHandlerand many more without that the enduser has to worry about it (which are however overrideable by injection).
• javax.faces.context.ExternalContext, which internally usesServletContext,HttpSession,HttpServletRequest,HttpServletResponse, etc.

Flyweight^{_{(recognizeable by creational methods returning a cached instance, a bit the "multiton" idea)}}

• java.lang.Integer#valueOf(int)(also onBoolean,Byte,Character,ShortandLong)

Proxy^{_{(recognizeable by creational methods which returns an implementation of given abstract/interface type which in turndelegates/usesadifferentimplementation of given abstract/interface type)}}

• java.lang.reflect.Proxy
• java.rmi.*, the whole API actually.

^{_{The Wikipedia example is IMHO a bit poor, lazy loading has actually completely nothing to do with the proxy pattern at all.}}

Behavioral patterns

Chain of responsibility^{_{(recognizeable by behavioral methods which (indirectly) invokes the same method inanotherimplementation ofsameabstract/interface type in a queue)}}

• java.util.logging.Logger#log()
• javax.servlet.Filter#doFilter()

Command^{_{(recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of adifferentabstract/interface type which has beenencapsulatedby the command implementation during its creation)}}

• All implementations ofjava.lang.Runnable
• All implementations ofjavax.swing.Action

Interpreter^{_{(recognizeable by behavioral methods returning astructurallydifferent instance/type of the given instance/type; note that parsing/formatting is not part of the pattern, determining the pattern and how to apply it is)}}

• java.util.Pattern
• java.text.Normalizer
• All subclasses ofjava.text.Format
• All subclasses ofjavax.el.ELResolver

Iterator^{_{(recognizeable by behavioral methods sequentially returning instances of adifferenttype from a queue)}}

• All implementations ofjava.util.Iterator(thus among others alsojava.util.Scanner!).
• All implementations ofjava.util.Enumeration

Mediator^{_{(recognizeable by behavioral methods taking an instance of different abstract/interface type (usually using the command pattern) which delegates/uses the given instance)}}

• java.util.Timer(allscheduleXXX()methods)
• java.util.concurrent.Executor#execute()
• java.util.concurrent.ExecutorService(theinvokeXXX()andsubmit()methods)
• java.util.concurrent.ScheduledExecutorService(allscheduleXXX()methods)
• java.lang.reflect.Method#invoke()

Memento^{_{(recognizeable by behavioral methods which internally changes the state of thewholeinstance)}}

• java.util.Date(the setter methods do that,Dateis internally represented by alongvalue)
• All implementations ofjava.io.Serializable
• All implementations ofjavax.faces.component.StateHolder

Observer (or Publish/Subscribe)^{_{(recognizeable by behavioral methods which invokes a method on an instance ofanotherabstract/interface type, depending on own state)}}

• java.util.Observer/java.util.Observable(rarely used in real world though)
• All implementations ofjava.util.EventListener(practically all over Swing thus)
• javax.servlet.http.HttpSessionBindingListener
• javax.servlet.http.HttpSessionAttributeListener
• javax.faces.event.PhaseListener

State^{_{(recognizeable by behavioral methods which changes its behaviour depending on the instance's state which can be controlled externally)}}

• javax.faces.lifecycle.LifeCycle#execute()(controlled byFacesServlet, the behaviour is dependent on current phase (state) of JSF lifecycle)

Strategy^{_{(recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of adifferentabstract/interface type which has beenpassed-inas method argument into the strategy implementation)}}

• java.util.Comparator#compare(), executed by among othersCollections#sort().
• javax.servlet.http.HttpServlet, theservice()and alldoXXX()methods takeHttpServletRequestandHttpServletResponseand the implementor has to process them (and not to get hold of them as instance variables!).
• javax.servlet.Filter#doFilter()

Template method^{_{(recognizeable by behavioral methods which already have a "default" behaviour definied by an abstract type)}}

• All non-abstract methods ofjava.io.InputStream,java.io.OutputStream,java.io.Readerandjava.io.Writer.
• All non-abstract methods ofjava.util.AbstractList,java.util.AbstractSetandjava.util.AbstractMap.
• javax.servlet.http.HttpServlet, all thedoXXX()methods by default sends a HTTP 405 "Method Not Allowed" error to the response. You're free to implement none or any of them.

Visitor^{_{(recognizeable by twodifferentabstract/interface types which has methods definied which takes each theotherabstract/interface type; the one actually calls the method of the other and the other executes the desired strategy on it)}}

• javax.lang.model.element.AnnotationValueandAnnotationValueVisitor
• javax.lang.model.element.ElementandElementVisitor
• javax.lang.model.type.TypeMirrorandTypeVisitor

（全文完）