Java 装饰模式讲解和代码示例

装饰是一种结构设计模式，允许你通过将对象放入特殊封装对象中来为原对象增加新的行为。

由于目标对象和装饰器遵循同一接口，因此你可用装饰来对对象进行无限次的封装。结果对象将获得所有封装器叠加而来的行为。

进一步了解装饰模式

复杂度：

流行度：

使用示例： 装饰在 Java 代码中可谓是标准配置，尤其是在与流式加载相关的代码中。

Java 核心程序库中有一些关于装饰的示例：

java.io.InputStream、 OutputStream、 Reader 和 Writer 的所有代码都有以自身类型的对象作为参数的构造函数。
java.util.Collections； checkedXXX()、 synchronizedXXX() 和 unmodifiableXXX() 方法。
javax.servlet.http.HttpServletRequestWrapper 和 HttpServletResponseWrapper

识别方法： 装饰可通过以当前类或对象为参数的创建方法或构造函数来识别。

编码和压缩装饰

本例展示了如何在不更改对象代码的情况下调整其行为。

最初的业务逻辑类仅能读取和写入纯文本的数据。此后，我们创建了几个小的封装器类，以便在执行标准操作后添加新的行为。

第一个封装器负责加密和解密数据，而第二个则负责压缩和解压数据。

你甚至可以让这些封装器嵌套封装以将它们组合起来。

decorators

decorators/DataSource.java: 定义了读取和写入操作的通用数据接口

package refactoring_guru.decorator.example.decorators;

public interface DataSource {
    void writeData(String data);

    String readData();
}

decorators/FileDataSource.java: 简单数据读写器

package refactoring_guru.decorator.example.decorators;

import java.io.*;

public class FileDataSource implements DataSource {
    private String name;

    public FileDataSource(String name) {
        this.name = name;
    }

    @Override
    public void writeData(String data) {
        File file = new File(name);
        try (OutputStream fos = new FileOutputStream(file)) {
            fos.write(data.getBytes(), 0, data.length());
        } catch (IOException ex) {
            System.out.println(ex.getMessage());
        }
    }

    @Override
    public String readData() {
        char[] buffer = null;
        File file = new File(name);
        try (FileReader reader = new FileReader(file)) {
            buffer = new char[(int) file.length()];
            reader.read(buffer);
        } catch (IOException ex) {
            System.out.println(ex.getMessage());
        }
        return new String(buffer);
    }
}

decorators/DataSourceDecorator.java: 抽象基础装饰

package refactoring_guru.decorator.example.decorators;

public abstract class DataSourceDecorator implements DataSource {
    private DataSource wrappee;

    DataSourceDecorator(DataSource source) {
        this.wrappee = source;
    }

    @Override
    public void writeData(String data) {
        wrappee.writeData(data);
    }

    @Override
    public String readData() {
        return wrappee.readData();
    }
}

decorators/EncryptionDecorator.java: 加密装饰

package refactoring_guru.decorator.example.decorators;

import java.util.Base64;

public class EncryptionDecorator extends DataSourceDecorator {

    public EncryptionDecorator(DataSource source) {
        super(source);
    }

    @Override
    public void writeData(String data) {
        super.writeData(encode(data));
    }

    @Override
    public String readData() {
        return decode(super.readData());
    }

    private String encode(String data) {
        byte[] result = data.getBytes();
        for (int i = 0; i < result.length; i++) {
            result[i] += (byte) 1;
        }
        return Base64.getEncoder().encodeToString(result);
    }

    private String decode(String data) {
        byte[] result = Base64.getDecoder().decode(data);
        for (int i = 0; i < result.length; i++) {
            result[i] -= (byte) 1;
        }
        return new String(result);
    }
}

decorators/CompressionDecorator.java: 压缩装饰

package refactoring_guru.decorator.example.decorators;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Base64;
import java.util.zip.Deflater;
import java.util.zip.DeflaterOutputStream;
import java.util.zip.InflaterInputStream;

public class CompressionDecorator extends DataSourceDecorator {
    private int compLevel = 6;

    public CompressionDecorator(DataSource source) {
        super(source);
    }

    public int getCompressionLevel() {
        return compLevel;
    }

    public void setCompressionLevel(int value) {
        compLevel = value;
    }

    @Override
    public void writeData(String data) {
        super.writeData(compress(data));
    }

    @Override
    public String readData() {
        return decompress(super.readData());
    }

    private String compress(String stringData) {
        byte[] data = stringData.getBytes();
        try {
            ByteArrayOutputStream bout = new ByteArrayOutputStream(512);
            DeflaterOutputStream dos = new DeflaterOutputStream(bout, new Deflater(compLevel));
            dos.write(data);
            dos.close();
            bout.close();
            return Base64.getEncoder().encodeToString(bout.toByteArray());
        } catch (IOException ex) {
            return null;
        }
    }

    private String decompress(String stringData) {
        byte[] data = Base64.getDecoder().decode(stringData);
        try {
            InputStream in = new ByteArrayInputStream(data);
            InflaterInputStream iin = new InflaterInputStream(in);
            ByteArrayOutputStream bout = new ByteArrayOutputStream(512);
            int b;
            while ((b = iin.read()) != -1) {
                bout.write(b);
            }
            in.close();
            iin.close();
            bout.close();
            return new String(bout.toByteArray());
        } catch (IOException ex) {
            return null;
        }
    }
}

Demo.java: 客户端代码

package refactoring_guru.decorator.example;

import refactoring_guru.decorator.example.decorators.*;

public class Demo {
    public static void main(String[] args) {
        String salaryRecords = "Name,Salary\nJohn Smith,100000\nSteven Jobs,912000";
        DataSourceDecorator encoded = new CompressionDecorator(
                                         new EncryptionDecorator(
                                             new FileDataSource("out/OutputDemo.txt")));
        encoded.writeData(salaryRecords);
        DataSource plain = new FileDataSource("out/OutputDemo.txt");

        System.out.println("- Input ----------------");
        System.out.println(salaryRecords);
        System.out.println("- Encoded --------------");
        System.out.println(plain.readData());
        System.out.println("- Decoded --------------");
        System.out.println(encoded.readData());
    }
}

OutputDemo.txt: 执行结果

- Input ----------------
Name,Salary
John Smith,100000
Steven Jobs,912000
- Encoded --------------
Zkt7e1Q5eU8yUm1Qe0ZsdHJ2VXp6dDBKVnhrUHtUe0sxRUYxQkJIdjVLTVZ0dVI5Q2IwOXFISmVUMU5rcENCQmdxRlByaD4+
- Decoded --------------
Name,Salary
John Smith,100000
Steven Jobs,912000