【Java】GoF-23种设计模式在Java核心类库中的应用

Creational patterns（创建型模式） Abstract factory（抽象工厂模式）

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（建造者模式）

recognizeable by creational methods returning the instance itself.

• java.lang.StringBuilder#append() (unsynchronized)
• java.lang.StringBuffer#append() (synchronized)
• java.nio.ByteBuffer#put() (also on CharBuffer, ShortBuffer, IntBuffer, LongBuffer, FloatBuffer and DoubleBuffer)
• javax.swing.GroupLayout.Group#addComponent()
• All implementations of java.lang.Appendable
• java.util.stream.Stream.Builder

Factory method（工厂模式）

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)
• java.util.EnumSet#of()
• javax.xml.bind.JAXBContext#createMarshaller() and other similar methods

Prototype（原型模式）

recognizeable by creational methods returning a different instance of itself with the same properties.

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

Singleton（单例模式）

recognizeable by creational methods returning the same instance (usually of itself) everytime.

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

Structural patterns（结构型模式） Adapter（适配器模式）

recognizeable by creational methods taking an instance of different abstract/interface type and returning an implementation of own/another abstract/interface type which decorates/overrides the given instance.

• java.util.Arrays#asList()
• java.util.Collections#list()
• java.util.Collections#enumeration()
• java.io.InputStreamReader(InputStream) (returns a Reader)
• java.io.OutputStreamWriter(OutputStream) (returns a Writer)
• javax.xml.bind.annotation.adapters.XmlAdapter#marshal() and #unmarshal()

Bridge（桥接模式）

recognizeable by creational methods taking an instance of different abstract/interface type and returning an implementation of own abstract/interface type which delegates/uses the given instance.

• None comes to mind yet. A fictive example would be new LinkedHashMap(LinkedHashSet, List) which returns an unmodifiable linked map which doesn’t clone the items, but uses them. The java.util.Collections#newSetFromMap() and singletonXXX() methods however comes close.

Composite（组合模式）

recognizeable by behavioral methods taking an instance of same abstract/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（装饰器模式）

recognizeable by creational methods taking an instance of same abstract/interface type which adds additional behaviour.

• All subclasses of java.io.InputStream, OutputStream, Reader and Writer have a constructor taking an instance of same type.
• java.util.Collections, the checkedXXX(), synchronizedXXX() and unmodifiableXXX() methods.
• javax.servlet.http.HttpServletRequestWrapper and HttpServletResponseWrapper
• javax.swing.JScrollPane

Facade（外观模式）

recognizeable by behavioral methods which internally uses instances of different independent abstract/interface types.

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

Flyweight（享元模式）

recognizeable by creational methods returning a cached instance, a bit the “multiton” idea.

• java.lang.Integer#valueOf(int) (also on Boolean, Byte, Character, Short, Long and BigDecimal)

Proxy（代理模式）

recognizeable by creational methods which returns an implementation of given abstract/interface type which in turn delegates/uses a different implementation of given abstract/interface type.

• java.lang.reflect.Proxy
• java.rmi.*
• javax.ejb.EJB (explanation here)
• javax.inject.Inject (explanation here)
• javax.persistence.PersistenceContext

Behavioral patterns（行为型模式） Chain of responsibility（责任链模式）

recognizeable by behavioral methods which (indirectly) invokes the same method in another implementation of same abstract/interface type in a queue.

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

Command（命令模式）

recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of a different abstract/interface type which has been encapsulated by the command implementation during its creation.

• All implementations of java.lang.Runnable
• All implementations of javax.swing.Action

Interpreter（解释器模式）

recognizeable by behavioral methods returning a structurally different 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 of java.text.Format
• All subclasses of javax.el.ELResolver

Iterator（迭代器模式）

recognizeable by behavioral methods sequentially returning instances of a different type from a queue.

• All implementations of java.util.Iterator (thus among others also java.util.Scanner!).
• All implementations of java.util.Enumeration

Mediator（中介者模式）

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 (all scheduleXXX() methods)
• java.util.concurrent.Executor#execute()
• java.util.concurrent.ExecutorService (the invokeXXX() and submit() methods)
• java.util.concurrent.ScheduledExecutorService (all scheduleXXX() methods)
• java.lang.reflect.Method#invoke()

Memento（备忘录模式）

recognizeable by behavioral methods which internally changes the state of the whole instance.

• java.util.Date (the setter methods do that, Date is internally represented by a long value)
• All implementations of java.io.Serializable
• All implementations of javax.faces.component.StateHolder

Observer(or Publish/Subscribe)（观察者模式）

recognizeable by behavioral methods which invokes a method on an instance of another abstract/interface type, depending on own state.

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

State（状态模式）

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 by FacesServlet, the behaviour is dependent on current phase (state) of JSF lifecycle)

Strategy（策略模式）

recognizeable by behavioral methods in an abstract/interface type which invokes a method in an implementation of a different abstract/interface type which has been passed-in as method argument into the strategy implementation.

• java.util.Comparator#compare(), executed by among others Collections#sort().
• javax.servlet.http.HttpServlet, the service() and all doXXX() methods take HttpServletRequest and HttpServletResponse and the implementor has to process them (and not to get hold of them as instance variables!).
• javax.servlet.Filter#doFilter()

Template method（模板模式）

recognizeable by behavioral methods which already have a “default” behaviour defined by an abstract type.

• All non-abstract methods of java.io.InputStream, java.io.OutputStream, java.io.Reader and java.io.Writer.
• All non-abstract methods of java.util.AbstractList, java.util.AbstractSet and java.util.AbstractMap.
• javax.servlet.http.HttpServlet, all the doXXX() 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 （访问者模式）

recognizeable by two different abstract/interface types which has methods defined which takes each the other abstract/interface type; the one actually calls the method of the other and the other executes the desired strategy on it.

• javax.lang.model.element.AnnotationValue and AnnotationValueVisitor
• javax.lang.model.element.Element and ElementVisitor
• javax.lang.model.type.TypeMirror and TypeVisitor
• java.nio.file.FileVisitor and SimpleFileVisitor
• javax.faces.component.visit.VisitContext and VisitCallback

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