params

     ①params={"username"}, 请求参数中必须有一个username,否则就会404, 比如 http://localhost:8083/1springmvc_helloworld/hello2?username=123

// 请求的参数中必须带一个username
	@RequestMapping(value="/hello2", params={"username"})
	public String hello2(){
		System.out.println("调用了hello2...");
		return "success";		
	}

   ② params={"!username"}, 加了一个感叹号,比如请求参数中不能带有username参数,否则就会404

// 请求的参数中必须带一个username
	@RequestMapping(value="/hello2", params={"!username"})
	public String hello2(){
		System.out.println("调用了hello2...");
		return "success";		
	}

  ③params={"username=123"},表示请求参数中必须有username, 并且username=123,这样才能映射到这个方法。 比如 http://localhost:8083/1springmvc_helloworld/hello2?username=123

@RequestMapping(value="/hello2", params={"username=123"})
	public String hello2(){
		System.out.println("调用了hello2...");
		return "success";		
	}

 ④params={"username!=123"}  加了一个感叹号,表示请求参数中可以不带参数username,或者带username,但username的值不能等于123

@RequestMapping(value="/hello2", params={"username!=123"})
	public String hello2(){
		System.out.println("调用了hello2...");
		return "success";		
	}

 ⑤. params={"username=123","pwd","!age"},  必须满足多个参数才能映射该方法,参数必须带有username和pwd,不能带有age,并且username必须等于123

@RequestMapping(value="/hello2", params={"username=123","pwd","!age"})
	public String hello2(){
		System.out.println("调用了hello2...");
		return "success";		
	}