设计模式（7）——Bridge模式（桥接）

梦里梦外; 2022-08-08 06:29 184阅读 0赞

### Bridge模式的作用 ###

Bridge模式就是把抽象部分与其实现部分分离，使它们都可以独立地变化。

![Image 1][]

[![Bridge UML class diagram.svg][]][Bridge UML class diagram.svg 1]

### Bridge模式的适用性 ###

1. 不希望在抽象和它的实现部分之间有一个固定的绑定关系。

2. 类的抽象以及它的实现都应该可以通过生成子类的方法加以扩充。

3. 对一个抽象的实现部分的修改应对客户不产生影响，即客户的代码不必重新编译

4. 想在多个对象间共享实现，但同时要求客户并不知道这一点。

### Bridge模式的示例 ###

从文件或数据库中读取数据

![Image 1][]

public interface Persistence {
    
    	/**
    	 * @param object
    	 * @return returns objectID
    	 */
    	public String persist(Object object);
    	
    	/**
    	 * 
    	 * @param objectId
    	 * @return persisted Object
    	 */
    	public Object findById(String objectId);
    	
    	/**
    	 * 
    	 * @param id
    	 */
    	public void deleteById(String id);
    	
    	
    }

public class PersistenceImp implements Persistence {
    
    	private PersistenceImplementor implementor = null;
    	
    	public PersistenceImp(PersistenceImplementor imp) {
    		
    		this.implementor = imp;
    		
    	}
    	
    	@Override
    	public void deleteById(String id) {
    
    		implementor.deleteObject(Long.parseLong(id));
    		
    	}
    
    	@Override
    	public Object findById(String objectId) {
    		
    		return implementor.getObject(Long.parseLong(objectId));
    	}
    
    	
    	
    	@Override
    	public String persist(Object object) {
    		
    		return Long.toString(implementor.saveObject(object));
    		
    
    	}
    
    }

/**
     * Implementor Interface 
     */
    public interface PersistenceImplementor {
    
    	public long saveObject(Object object);
    	
    	public void deleteObject(long objectId);
    	
    	public Object getObject(long objectId);
    	
    	
    }

从文件系统读写数据

import java.io.File;
    
    /**
     * Concrete Implementor 
     *
     */
    public class FileSystemPersistenceImplementor implements PersistenceImplementor{
    
    	@Override
    	public void deleteObject(long objectId) {
    
    		File f = new File("/persistence/"+Long.toString(objectId));
    		
    		f.delete();
    		
    		return;
    	}
    
    	@Override
    	public Object getObject(long objectId) {
    		
    		File f = new File("/persistence/"+Long.toString(objectId));
    		
    		return readObjectFromFile(f);
    		
    		
    	}
    
    	private Object readObjectFromFile(File f) {
    
    		// open file 
    		// and load object 
    		//return the object
    		return null;
    	}
    
    	@Override
    	public long saveObject(Object object) {
    
    		long fileId = System.currentTimeMillis();
    		
    		// open file 
    		File f = new File("/persistence/"+Long.toString(fileId));
    	
    		// write file to Streanm 
    		
    		writeObjectToFile(f,object);
    		
    		
    		return fileId;
    	
    	}
    
    	private void writeObjectToFile(File f, Object object) {
    
    		// serialize object and write it to file
    		
    	}
    
    }

从数据库读写数据：

public class DabatasePersistenceImplementor implements PersistenceImplementor{
    
    	public DabatasePersistenceImplementor() {
    	
    		// load database driver
    		
    		
    	}
    	
    	@Override
    	public void deleteObject(long objectId) {
    
    		// open database connection
    		// remove record
    		
    	}
    
    	@Override
    	public Object getObject(long objectId) {
    
    		// open database connection 
    		// read records
    		// create object from record 
    		return null;
    	}
    
    	@Override
    	public long saveObject(Object object) {
    
    		// open database connection 
    		// create records for fields inside the object
    
    		return 0;
    	}
    
    }

Persistence API driver。注意，实际的实现并不影响客户端代码。

public class PersistenceFrameworkDriver {
    	
    	public static void main(String[] args) {
    		
    		
    		// this program needs a persistence framework
    		// at runtime  an implementor is chosen between file system implementation and
    		//database implememtor , depending on existence of databse drivers 在运行时，在数据库或文件实现类之间进行选择。
    		
    		
    		
    		PersistenceImplementor implementor = null;
    		
    		if(databaseDriverExists()){
    			
    			implementor = new DabatasePersistenceImplementor();
    			
    		}else{
    			
    			implementor = new FileSystemPersistenceImplementor();		
    		}
    		
    		Persistence persistenceAPI = new PersistenceImp(implementor);
    		
    		Object o = persistenceAPI.findById("12343755");
    		
    		
    		// do changes to the object 
    		
    		// then persist 
    		
    		persistenceAPI.persist(o);
    		
    		
    		// can also change implementor
    		persistenceAPI = new PersistenceImp(new DabatasePersistenceImplementor()); 
    
    		persistenceAPI.deleteById("2323");
    		
    		
    	}
    
    	private static boolean databaseDriverExists() {
    	
    		return false;
    	}
    }

链接：http://www.oodesign.com/bridge-pattern.html

[Image 1]: 
[Bridge UML class diagram.svg]: /images/20220805/879488b932e3480f98ac25daf339cd1e.png
[Bridge UML class diagram.svg 1]: http://en.wikipedia.org/wiki/File:Bridge_UML_class_diagram.svg