See how I store integer data in the map ＜ string, string ＞ set

One 、 Antecedents feed

A few days ago, the test gave me a defect , Prompt type conversion error , But I can't figure out what the problem is with the corresponding code

1、 Defect background

Abstract the model roughly , Other colleagues put the front desk data into a map in , And then put this map To json The string is passed to the background , And then I'll put this json The string is deserialized into a map object , Finally put this map Object passed to me .

2、 The error report is as follows ：

Just read the error message , In fact, I am still very happy , This is obviously a type conversion exception , It must be that I will Integer The number of type is converted to a String String of type , Keep looking down

3、 Where the code reports an error

The following method is used to locate the error location , Then I fell into a deep thought ,String turn Integer？ What kind of type conversion exception are you ？ I am not two String？

4、 This method is only a simple method for judging the blank

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Two 、 Looking for problems

1、 Check the receiving parameters in the log

I'm glad , Fortunately, I printed the received parameters in the log . It turned out that when I typed the log, I sent it directly map, There is no custom serialization for it , So that I can't see the corresponding type of data （ It will be explained later , This data type is important ）, Next time you have to remember to serialize

2、 Observe the parameters passed from the foreground to the background

but , I can only view the corresponding parameters at the front desk , Finally, I found it in the parameters of the foreground . It is found that a parameter does not look like a string

3、 Execute the judgment method again

Then I use numeric arguments to execute StringUtils.isNotBlank() Method , I can only say that this mistake is right .

But here comes a new problem ：

1. If it is directly digital compilation, it will not pass ！
2. Even if it is Object Parameters of type , I still need a forced transfer here , But my code is not forced to turn ！
3. Also, the parameter generic type of the method I reported an error is Map<String,String>, Generic types are limited to String Class cannot receive Integer Type parameter ！ Really? ？

4、 So after reading my colleague's code , Locate the problem again

In fact, the work of colleagues is to give the receptionist json character string , Deserialize to a Map<String,String> The collection of . Then the problem can only be in this deserialization , There are still numbers when they are passed in , How come there are no numbers ？

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3、 ... and 、 Verification questions

1、 Deserialize your own test

Simulate the whole process code

public class Map Generic compatibility  {
    
	public static void main(String[] args) {
    
		//  Simulate the data transmitted from the foreground to the background 
		Map<String, Object> map = new HashMap<>();
		map.put("key1", 1);
		map.put("key2", "a");

		String jsonStr = "";
		jsonStr = JSON.toJSONString(map);
		//  To the backstage json character string 
		System.out.println(jsonStr);

		//  Deserialize the string 
		Map<String, String> params = JSON.parseObject(jsonStr, HashMap.class);

		for (Map.Entry<String, String> entry : params.entrySet()) {
    
			if (entry.getValue() instanceof String) {
    
			} else {
    
				System.out.println(" Not a string type ");
			}
			System.out.println(entry.getValue());
		}
	}
}

We were surprised to find out , I didn't report a mistake , He didn't report a mistake , He didn't report a mistake ,Integer The transformation of is to String No problem , And when we finally got the data, it was still Integer type , I'm a good guy .

Then I thought about it json Deserialization is done through reflection .

2、 Reflection revalidation

It can actually , I use reflection to Map<String,String> Add data to the object of , And then traverse the map object , But I found that value It's a Integer type .

Then everything makes sense

Verification code ：

public class TestReflect {
    
	public static void main(String[] args) throws Exception {
    
		Map<String, String> map = new HashMap<>();

		Method put = map.getClass().getMethod("put", Object.class, Object.class);
		put.invoke(map, "key1", 1);
		put.invoke(map, "key2", "value2");
		System.out.println(JSON.toJSON(map));

		for (Map.Entry<String, String> entry : map.entrySet()) {
    
			if (entry.getValue() instanceof String) {
    
			} else {
    
				System.out.println(" The key It's other types ：" + entry.getValue().getClass());
			}
		}
	}
}

3、 Conclusion

The final conclusion is , When colleagues transfer all the parameters of the foreground to the background , It contains a value by Integer Data of type , That analogy is put into a Map<String,Object> Of map Collection , In response to the map After serialization , It becomes an ordinary json character string .

The key to the problem lies in the position of inverse sequence , In response to the json When a string is deserialized , There's a problem . But this is a runtime exception . Because of our json The string contains Integer Parameters of type , But the deserialized object is a String type , According to the previous demonstration , When using reflection for assignment , No errors are reported , So that everything is calm .

But when we are using the corresponding parameters , Here's the problem , Because we only recognize deserialized String Type of map, But what we actually got was Integer, That led to subsequent errors .

How should we use it ？

In my opinion, if we are not sure whether there are multiple types of parameters , have access to Object Type of map Receive the set data from the inverse sequence , Do not use the forced conversion form when obtaining the corresponding parameters , Instead, use more friendly String.valueOf Get the corresponding content （ The bottom layer is to call the wrapper type toString Method ）