摘要：測(cè)試類設(shè)計(jì)模式裝飾者模式工廠模式只能讀啦會(huì)報(bào)錯(cuò)只讀異?？梢哉_運(yùn)行第二部分定義抽象組件是具體組件和抽象裝飾類的共同父類，聲明了在具體組件中實(shí)現(xiàn)的方法。

本篇文章分為四個(gè)部分：第一部分會(huì)舉一個(gè)例子引出裝飾者模式，讓讀者對(duì)裝飾者模式有個(gè)感官上的認(rèn)識(shí)；第二部分會(huì)給出裝飾者模式的定義（當(dāng)然我們主要不是來(lái)背定義，就當(dāng)做積累專業(yè)名詞來(lái)記吧，我個(gè)人是很不喜歡下定義的）；第三部分，我會(huì)拿jdk中兩個(gè)使用裝飾者模式的例子進(jìn)行分析，讓讀者在學(xué)習(xí)模式的同時(shí)熟悉一下jdk源碼。第四部分，我會(huì)結(jié)合裝飾者模式理清楚java I/O各種流之間的關(guān)系（說(shuō)實(shí)話，工作中用得最多，但是記了又忘，因?yàn)镮/O類實(shí)在太多）后面學(xué)習(xí)到其他框架的時(shí)候再補(bǔ)充。

假如有這樣的需求：要求實(shí)現(xiàn)只能夠讀的List,要是你來(lái)完成這個(gè)任務(wù)你會(huì)如何做？
看下面代碼實(shí)現(xiàn)：

package decorate;
import java.util.*;

public class ReadOnlyList implements List{

    private List target;

    public ReadOnlyList(List 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 iterator() {
        return target.iterator();
    }

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

    @Override
    public  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 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 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 listIterator() {
        return target.listIterator();
    }

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

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

}

這里的set,add,remove和addAll等操作都被限制了，只能夠讀。
測(cè)試類

package decorate;

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

public class ReadOnlyListMain {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add("設(shè)計(jì)模式");
        list.add("裝飾者模式");
        list.add("工廠模式");
        ReadOnlyList onlyList = new ReadOnlyList<>(list);
//        onlyList.add("只能讀啦"); // 會(huì)報(bào)錯(cuò)只讀異常
        System.out.println(onlyList.size()); // 可以正確運(yùn)行
    }
}

Component(抽象組件): 是具體組件和抽象裝飾類的共同父類，聲明了在具體組件中實(shí)現(xiàn)的方法。比如第一部分的List
ConcreteComponent（具體組件）: 抽象組件的子類，實(shí)現(xiàn)抽象組件的方法，裝飾器可以給他加新的功能。比如ReadOnlyList
Decorator（抽象裝飾者）: 抽象組件的子類，用來(lái)裝飾具體組件或者裝飾其他裝飾組件
ConcreteDecorator（具體裝飾者):抽象裝飾類的子類
具體的可以看下面的結(jié)構(gòu)圖：

第一部分我們引出裝飾者模式，ReadOnlyList 只讀List將List給包裝起來(lái)，提供了只讀的功能，jdk Collection中也有個(gè)類似的實(shí)現(xiàn):
public static List unmodifiableList(List list)

讓我來(lái)看看它的源碼：

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

我們進(jìn)入 UnmodifiableList

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

        final List list;

        UnmodifiableList(List 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 c) {
            throw new UnsupportedOperationException();
        }

        @Override
        public void replaceAll(UnaryOperator operator) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void sort(Comparator c) {
            throw new UnsupportedOperationException();
        }

        public ListIterator listIterator()   {return listIterator(0);}

        public ListIterator listIterator(final int index) {
            return new ListIterator() {
                private final ListIterator 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 action) {
                    i.forEachRemaining(action);
                }
            };
        }

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

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

你們發(fā)現(xiàn)了啥，沒(méi)錯(cuò)，就是它：addAll、replaceAll和sort等都拋出 UnsupportedOperationException異常，說(shuō)明只支持讀。

還沒(méi)結(jié)束，再舉個(gè)例子，Collections中還有將線程不安全的集合轉(zhuǎn)換成線程安全的集合synchronizedList，也就是使用裝飾者模式，本質(zhì)上就是在方法加上synchronized 同步鎖

讓我們看源碼：

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

老規(guī)矩，進(jìn)入SynchronizedList

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

        final List list;

        SynchronizedList(List list) {
            super(list);
            this.list = list;
        }
        SynchronizedList(List 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 c) {
            synchronized (mutex) {return list.addAll(index, c);}
        }

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

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

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

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

嘿嘿，我們現(xiàn)在已經(jīng)學(xué)會(huì)從設(shè)計(jì)模式的角度看jdk源碼啦，開(kāi)心。

我們來(lái)看看Java I/O中的流圖

我們以輸入流來(lái)分析，首先InputStream 相當(dāng)于我們的抽象組件，F(xiàn)ileInputStream、StringBufferInputStream、ByteArrayInputStream都是可以被裝飾者包裝起來(lái)的組件，F(xiàn)ilterInputStream相當(dāng)于抽象裝飾者,PushbackInputStream、BufferedInputStream、DataInputStream和LineNumberInputStream都是具體的裝
飾者

他們可以這樣用: 一層裝飾一層

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

如下圖所示：

有了這樣的思路，我們以后想編寫自己的I/O，給流增加新的特性，我們就可以繼承FilterInputStream，

package decorate;

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

/**
 * 將輸入的流內(nèi)所有的大寫字符轉(zhuǎn)成小寫字符
 */
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;
    }
}

測(cè)試類：把文件路徑替換成你自己的路徑

package decorate;

import java.io.*;

/**
 * 測(cè)試類
 */
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();
        }

    }
}