装饰器模式在开源代码中的应用

r囧r小猫 2023-03-02 05:18 1阅读 0赞

装饰器模式的作用：动态的给对象增加一些职责，即增加其额外的功能。

**案例**

装饰器模式比较经典的应用就是 JDK 中的 java.io 包下，InputStream、OuputStream、Reader、Writer 及它们的子类。

以 InputStream 为例

*  FileInputStream 是 InputStream 的子类，用来读取文件字节流
 *  BufferedInputStream 是 InputStream 的子类的子类，可缓存的字节流
 *  DataInputStream 也是 InputStream 的子类的子类，可直接读取 Java 基本类型的字节流

如果希望提供一个可以读取文件 + 可缓存的字节流，使用继承方式，就需要派生 FileBufferedInputStream；

如果希望提供一个可以读取文件 + 直接读取基本类型的字节流，使用继承方式，就需要派生 FileDataInputStream。

字节流功能的增强还包括支持管道 pipe、字节数组 bytearray、字节对象 object、字节流字符流的转换 等维度，如果用继承方式，那类的层级与种类会多到爆炸。

为了解决问题，这边就使用了装饰器模式。

InputStream 源码简化

public abstract class InputStream implements Closeable {
        public int read(byte b[], int off, int len) throws IOException {}
    }

FileInputStream 源码简化

public class FileInputStream extends InputStream {
        public FileInputStream(String name) throws FileNotFoundException {
            this(name != null ? new File(name) : null);
        }
    
        public int read(byte b[], int off, int len) throws IOException {
            return readBytes(b, off, len);
        }
    }

BufferedInputStream 源码精简，继承自 FilterInputStream ，改为 InputStream

public class BufferedInputStream extends InputStream {
        protected volatile InputStream in;
    
        public BufferedInputStream(InputStream in) {
            this.in = in;
        }
    
        public synchronized int read(byte b[], int off, int len) throws IOException {
        ...
        }
    }

DataInputStream 源码精简，继承自 FilterInputStream ，改为 InputStream

public class DataInputStream extends InputStream {
        protected volatile InputStream in;
    
        public DataInputStream(InputStream in) {
            this.in = in;
        }
    
        public final int read(byte b[], int off, int len) throws IOException {
            return in.read(b, off, len);
        }
    
    }

组合各种类型的字节流，使用

//读取文件 + 可缓存的字节流
    new BufferedInputStream(new FileInputStream("E:/a.txt"));
    
    //读取文件 + 直接读取基本类型的字节流
    new DataInputStream(new FileInputStream("E:/a.txt"));

装饰器模式有两个特点：

*  装饰器类和原始类继承同样的父类，可以通过构造方法对原始类“嵌套”多个装饰器类
 *  装饰器类是对功能的增强

PS：BufferedInputStream、DataInputStream 都继承自 FilterInputStream，FilterInputStream 对 InputStream 进行了公共实现，避免 BufferedInputStream、DataInputStream 都各自进行相同的实现。

--------------------

### 【Java学习资源】整理推荐 ###

*  [装饰器模式在开源代码中的应用][Link 1]
 *  [桥接模式在开源代码中的应用][Link 2]
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 *  [工厂模式在开源代码中的应用][Link 7]
 *  [单例模式在开源代码中的应用][Link 8]
 *  [编码规范][Link 9]
 *  [设计模式][Link 10]
 *  [重构][Link 11]
 *  [设计原则][Link 12]
 *  [面向对象到底是什么][Link 13]
 *  [代码质量有哪些评判标准？][Link 14]

--------------------

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