设计模式系列之装饰者模式

前言

本篇文章分为四个部分：第一部分会举一个例子引出装饰者模式，让读者对装饰者模式有个感官上的认识；第二部分会给出装饰者模式的定义（当然我们主要不是来背定义，就当做积累专业名词来记吧，我个人是很不喜欢下定义的）；第三部分，我会拿 jdk 中两个使用装饰者模式的例子进行分析，让读者在学习模式的同时熟悉一下 jdk 源码。第四部分，我会结合装饰者模式理清楚 java I/ O 各种流之间的关系（说实话，工作中用得最多，但是记了又忘，因为 I / O 类实在太多）后面学习到其他框架的时候再补充。

第一部分

假如有这样的需求：要求实现只能够读的 List, 要是你来完成这个任务你会如何做？
看下面代码实现：

package decorate;
import java.util.*;

public class ReadOnlyList<E> implements List<E>{

    private List<E> target;

    public ReadOnlyList(List<E> target) {super();
        this.target = target;
    }

    @Override
    public int size() {return target.size();
    }

    @Override
    public boolean isEmpty() {return target.isEmpty();
    }

    @Override
    public boolean contains(Object o) {return target.contains(o);
    }

    @Override
    public Iterator<E> iterator() {return target.iterator();
    }

    @Override
    public Object[] toArray() {return target.toArray();
    }

    @Override
    public <T> T[] toArray(T[] a) {return target.toArray(a);
    }

    @Override
    public boolean add(E e) {throw new RuntimeException("only read");
    }

    @Override
    public boolean remove(Object o) {throw new RuntimeException("only read");
    }

    @Override
    public boolean containsAll(Collection<?> c) {return target.containsAll(c);
    }

    @Override
    public boolean addAll(Collection<? extends E> c) {throw new RuntimeException("only read");
    }

    @Override
    public boolean removeAll(Collection<?> c) {throw new RuntimeException("only read");
    }

    @Override
    public boolean retainAll(Collection<?> c) {throw new RuntimeException("only read");
    }

    @Override
    public void clear() {throw new RuntimeException("only read");
    }

    @Override
    public boolean addAll(int index, Collection<? extends E> c) {throw new RuntimeException("only read");
    }

    @Override
    public E get(int index) {return target.get(index);
    }

    @Override
    public E set(int index, E element) {throw new RuntimeException("only read");
    }

    @Override
    public void add(int index, E element) {throw new RuntimeException("only read");
    }

    @Override
    public E remove(int index) {throw new RuntimeException("only read");
    }

    @Override
    public int indexOf(Object o) {return target.indexOf(o);
    }

    @Override
    public int lastIndexOf(Object o) {return target.lastIndexOf(o);
    }

    @Override
    public ListIterator<E> listIterator() {return target.listIterator();
    }

    @Override
    public ListIterator<E> listIterator(int index) {return target.listIterator(index);
    }

    @Override
    public List<E> subList(int fromIndex, int toIndex) {return target.subList(fromIndex, toIndex);
    }

}

这里的 set,add,remove 和 addAll 等操作都被限制了，只能够读。
测试类

package decorate;

import java.util.ArrayList;
import java.util.List;

public class ReadOnlyListMain {public static void main(String[] args) {List<String> list = new ArrayList<>();
        list.add("设计模式");
        list.add("装饰者模式");
        list.add("工厂模式");
        ReadOnlyList onlyList = new ReadOnlyList<>(list);
//        onlyList.add("只能读啦"); // 会报错只读异常
        System.out.println(onlyList.size()); // 可以正确运行
    }
}

第二部分 定义

Component(抽象组件): 是具体组件和抽象装饰类的共同父类，声明了在具体组件中实现的方法。比如第一部分的 List
ConcreteComponent（具体组件）: 抽象组件的子类，实现抽象组件的方法，装饰器可以给他加新的功能。比如 ReadOnlyList
Decorator（抽象装饰者）: 抽象组件的子类，用来装饰具体组件或者装饰其他装饰组件
ConcreteDecorator（具体装饰者): 抽象装饰类的子类
具体的可以看下面的结构图：

第三部分 jdk 例子分析

第一部分我们引出装饰者模式，ReadOnlyList 只读 List 将 List 给包装起来，提供了只读的功能，jdk Collection 中也有个类似的实现:
public static <T> List<T> unmodifiableList(List<? extends T> list)

让我来看看它的源码：

public static <T> List<T> unmodifiableList(List<? extends T> list) {
        return (list instanceof RandomAccess ?
                new UnmodifiableRandomAccessList<>(list) :
                new UnmodifiableList<>(list));
    }

我们进入 UnmodifiableList

 /**
     * @serial include
     */
    static class UnmodifiableList<E> extends UnmodifiableCollection<E>
                                  implements List<E> {
        private static final long serialVersionUID = -283967356065247728L;

        final List<? extends E> list;

        UnmodifiableList(List<? extends E> list) {super(list);
            this.list = list;
        }

        public boolean equals(Object o) {return o == this || list.equals(o);}
        public int hashCode()           {return list.hashCode();}

        public E get(int index) {return list.get(index);}
        public E set(int index, E element) {throw new UnsupportedOperationException();
        }
        public void add(int index, E element) {throw new UnsupportedOperationException();
        }
        public E remove(int index) {throw new UnsupportedOperationException();
        }
        public int indexOf(Object o)            {return list.indexOf(o);}
        public int lastIndexOf(Object o)        {return list.lastIndexOf(o);}
        public boolean addAll(int index, Collection<? extends E> c) {throw new UnsupportedOperationException();
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator<? super E> c) {throw new UnsupportedOperationException();
        }

        public ListIterator<E> listIterator()   {return listIterator(0);}

        public ListIterator<E> listIterator(final int index) {return new ListIterator<E>() {
                private final ListIterator<? extends E> i
                    = list.listIterator(index);

                public boolean hasNext()     {return i.hasNext();}
                public E next()              {return i.next();}
                public boolean hasPrevious() {return i.hasPrevious();}
                public E previous()          {return i.previous();}
                public int nextIndex()       {return i.nextIndex();}
                public int previousIndex()   {return i.previousIndex();}

                public void remove() {throw new UnsupportedOperationException();
                }
                public void set(E e) {throw new UnsupportedOperationException();
                }
                public void add(E e) {throw new UnsupportedOperationException();
                }

                @Override
                public void forEachRemaining(Consumer<? super E> action) {i.forEachRemaining(action);
                }
            };
        }

        public List<E> subList(int fromIndex, int toIndex) {return new UnmodifiableList<>(list.subList(fromIndex, toIndex));
        }

        
        private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new UnmodifiableRandomAccessList<>(list)
                    : this);
        }
    }

你们发现了啥，没错，就是它：addAll、replaceAll 和 sort 等都抛出 UnsupportedOperationException 异常，说明只支持读。

还没结束，再举个例子，Collections 中还有将线程不安全的集合转换成线程安全的集合 synchronizedList，也就是使用装饰者模式，本质上就是在方法加上 synchronized 同步锁

让我们看源码：

public static <T> List<T> synchronizedList(List<T> list) {
        return (list instanceof RandomAccess ?
                new SynchronizedRandomAccessList<>(list) :
                new SynchronizedList<>(list));
    }

老规矩，进入 SynchronizedList

/**
     * @serial include
     */
    static class SynchronizedList<E>
        extends SynchronizedCollection<E>
        implements List<E> {
        private static final long serialVersionUID = -7754090372962971524L;

        final List<E> list;

        SynchronizedList(List<E> list) {super(list);
            this.list = list;
        }
        SynchronizedList(List<E> list, Object mutex) {super(list, mutex);
            this.list = list;
        }

        public boolean equals(Object o) {if (this == o)
                return true;
            synchronized (mutex) {return list.equals(o);}
        }
        public int hashCode() {synchronized (mutex) {return list.hashCode();}
        }

        public E get(int index) {synchronized (mutex) {return list.get(index);}
        }
        public E set(int index, E element) {synchronized (mutex) {return list.set(index, element);}
        }
        public void add(int index, E element) {synchronized (mutex) {list.add(index, element);}
        }
        public E remove(int index) {synchronized (mutex) {return list.remove(index);}
        }

        public int indexOf(Object o) {synchronized (mutex) {return list.indexOf(o);}
        }
        public int lastIndexOf(Object o) {synchronized (mutex) {return list.lastIndexOf(o);}
        }

        public boolean addAll(int index, Collection<? extends E> c) {synchronized (mutex) {return list.addAll(index, c);}
        }

        public ListIterator<E> listIterator() {return list.listIterator(); // Must be manually synched by user
        }

        public ListIterator<E> listIterator(int index) {return list.listIterator(index); // Must be manually synched by user
        }

        public List<E> subList(int fromIndex, int toIndex) {synchronized (mutex) {return new SynchronizedList<>(list.subList(fromIndex, toIndex),
                                            mutex);
            }
        }

        @Override
        public void replaceAll(UnaryOperator<E> operator) {synchronized (mutex) {list.replaceAll(operator);}
        }
        @Override
        public void sort(Comparator<? super E> c) {synchronized (mutex) {list.sort(c);}
        }
private Object readResolve() {
            return (list instanceof RandomAccess
                    ? new SynchronizedRandomAccessList<>(list)
                    : this);
        }

嘿嘿，我们现在已经学会从设计模式的角度看 jdk 源码啦，开心。

第四部分 Java I/ O 流中的装饰者模式

我们来看看 Java I/ O 中的流图

我们以输入流来分析，首先 InputStream 相当于我们的抽象组件，FileInputStream、StringBufferInputStream、ByteArrayInputStream 都是可以被装饰者包装起来的组件，FilterInputStream 相当于抽象装饰者,PushbackInputStream、BufferedInputStream、DataInputStream 和 LineNumberInputStream 都是具体的装
饰者

他们可以这样用: 一层装饰一层

    InputStream ip = new DataInputStream(new BufferedInputStream(new FileInputStream(new File("/file_path"))));

如下图所示：

有了这样的思路，我们以后想编写自己的 I /O，给流增加新的特性，我们就可以继承 FilterInputStream，

package decorate;

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * 将输入的流内所有的大写字符转成小写字符
 */
public class LowerCaseInputStream extends FilterInputStream{public LowerCaseInputStream(InputStream in) {super(in);
    }

    @Override
    public int read() throws IOException {int c = super.read();
        if (c == -1) {return c;} else {return Character.toLowerCase((char)c);
        }
    }

    @Override
    public int read(byte[] b, int off, int len) throws IOException {int result = super.read(b, off, len);
        for (int i = off; i < off + result; i++) {b[i] = (byte) Character.toLowerCase((char)b[i]);
        }
        return result;
    }
}

测试类：把文件路径替换成你自己的路径

package decorate;

import java.io.*;

/**
 * 测试类
 */
public class MyInputStreamTest {public static void main(String[] args) {
        int c;
        try {InputStream in = new LowerCaseInputStream(new BufferedInputStream(new FileInputStream("file_path")));
            while((c = in.read()) >= 0) {System.out.println((char)c);
            }
        } catch (IOException e) {e.printStackTrace();
        }

    }
}