Java语言学习二十四（注解）

淡淡的烟草味﹌ 2023-07-20 12:10 21阅读 0赞

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## 常见注解 ##

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## 注解用法 ##

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## 如何定义注解 ##

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## 注解应用 ##

注解在Java开发框架中使用广泛，Spring，Hibernate，SpringMVC中都会用到注解配置框架，简化程序设计，提高开发效率。

定义自动创建对象注解

package com.gf.erp.spring;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target({ ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    public @interface MyService { 
    
    }

定义测试类

package com.gf.erp.spring;
    
    @MyService
    public class TestService { 
    
    }

容器类

package com.gf.erp.spring;
    
    import java.io.File;
    import java.util.ArrayList;
    import java.util.List;
    
    public class MyAppContext { 
    	private List<Object> beans = new ArrayList<Object>();
    	
    	public MyAppContext()
    	{ 
    		try
    		{ 
    			scan();
    		}
    		catch(Exception e)
    		{ 
    			e.printStackTrace();
    		}
    	}
    	
    	public Object getBean(Class clz)
    	{ 
    		for(Object obj:beans)
    		{ 
    			if(obj.getClass() == clz)
    				return obj;
    		}
    		return null;
    	}
    	
    	//扫描类路径，只要使用MyService注解标注的类，统一生成对象
    	private void scan() throws Exception
    	{ 
    		List<String> list = getClasses(this.getClass());
    		for(String className:list)
    		{ 
    			Class clz = Class.forName(className);
    			if(clz.isAnnotationPresent(MyService.class))
    			{ 
    				Object obj = clz.newInstance();
    				beans.add(obj);
    			}
    		}
    		
    	}
    	
    	//获取clz所在目录的所有类全称，不含.class文件扩展名
    	private List<String> getClasses(Class clz)
    	{ 
    		String file = clz.getTypeName().replace(".","/")+".class";
    		String path = clz.getClassLoader().getResource(file).toString();
    		int pos = path.indexOf(file);
    		String suffix = "file:/";
    		String classPath = path.substring(suffix.length(),pos);
    		List<String> rtn = getFullClassName(classPath);
    		return rtn;
    	}
    	
    	private List<String> getFullClassName(String path)
    	{ 
    		List<String> rtn = new ArrayList<String>();
    		File f = new File(path);
    		recursePath(rtn,f);
    		
    		//去掉前面目录前缀
    		List<String> result = new ArrayList<String>();
    		for(String str:rtn)
    		{ 
    			int pos = str.indexOf(path);
    			//截取前面目录，只保留包名和类名
    			str = str.substring(pos+path.length()+1);
    			//只扫描Class类
    			if(str.endsWith(".class"))
    			{ 
    				//去掉文件扩展名.class
    				str = str.substring(0,str.length()-6);
    				//将反斜线转化为.
    				str = str.replace("\\",".");
    				result.add(str);
    			}
    		}
    		
    		return result;
    	}
    	
    	//递归遍历目录
    	private void recursePath(List<String> rtn,File f)
    	{ 
    		File[] files = f.listFiles();
    		for(File f2:files)
    		{ 
    			if(f2.isFile())
    				rtn.add(f2.getAbsolutePath());
    			else
    				recursePath(rtn,f2);
    		}
    	}
    	
    	public static void main(String[] args)
    	{ 
    		MyAppContext app = new MyAppContext();
    		Object obj = app.getBean(TestService.class);
    		System.out.println("obj="+obj);
    	}
    }

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