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It is believed that many inexperienced people don't know what to do about how Tomcat handles request parameters. Therefore, this article summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.

The form with parameters in a url request (that is, the GET request) starts with a question mark after the request target, followed by parameter name-value pairs, and multiple name-value pairs are separated by a sum sign (&).

How are the parameters passed through URL parsed in Tomcat?

We usually get a parameter in Servlet by the following ways

String value = request.getParameter ("paramName")

We take it directly from the parameter name when we need it. What is the setting of this value? How do name-value pairs correspond?

We follow the getParameter method of this vine to touch the realization of this melon.

When we use the HttpServletRequest object, we have been using a facade object (RequestFacade), which uses the facade pattern in the design pattern, encapsulates some details in the HttpServletRequest, and only exposes some necessary API.

When the actual request is processed, its encapsulated request object is called.

The code for the getParameter method looks like this:

/ * Return the value of the specified request parameter, if any; otherwise

* return null. If there is more than one value defined

* return only the first one.

* @ param name Name of the desired request parameter

, /

Public String getParameter (String name) {

If (! parametersParsed) {

ParseParameters ()

}

Return coyoteRequest.getParameters () getParameter (name)

}

Each request will first determine whether the parameter has been parsed, and return directly if it has been parsed.

Protected void parseParameters () {

ParametersParsed = true; / / Note here, after parsing, it is set to true.

Parameters parameters = coyoteRequest.getParameters ()

Boolean success = false

Try {

/ / Set this every time in case limit has been changed via JMX

Parameters.setLimit (getConnector () .getMaxParameterCount ())

}

...

Parameters.handleQueryParameters ()

}

So, the configuration of this name-value pair, initialization, occurs when the getParameter method is called for the first time.

Further down, this handleQueryParameters is the specific method to deal with. Here we assume that the request for url is as follows:

Http://localhost:8080/test?abc=1&def=2

In the handleQueryParameters method, we observe through the debug interface.

Here parameters contains an attribute queryMB whose value happens to be the string we passed in. So the later parameter processing is based on this property.

After that, in the processParameter method of the class Parameter

We see that basically you iterate through each char in a string, encounter specific characters = and &, and then get the name and value before and after the equal sign from each index.

A special place in the middle is that when you encounter% and +, there are things like Chinese characters, which actually need to be escaped, so the processing is also carried out here.

After parsing, the name-value pair is stored in a data structure like ArrayList. Look at the following code

Public void addParameter (String key, String value) {

ArrayList values = paramHashValues.get (key)

If (values = = null) {

Values = new ArrayList (1)

ParamHashValues.put (key, values)

}

Values.add (value)

}

After the above method is executed, the code executes downwards, and the value of the parameters object has become like this:

Abc=1,\ ndef=2,\ n

Notice the two lines marked red and bold in the code above

Do you remember that if you have more than one parameter, only the first match is returned for those with the same name?

In the parameter request for a specific request, if the initial processing is not involved, the following code is executed. It is very simple to take the ArrayList of the corresponding key from Map, and if there is a value, take the first value from it.

Public String getParameter (String name) {

HandleQueryParameters ()

ArrayList values = paramHashValues.get (name)

If (values! = null) {

If (values.size () = = 0) {

Return ""

}

Return values.get (0); / / notice here that you are fulfilling the promise to return only the first one!

} else {

Return null

}

} after reading the above, have you mastered how Tomcat handles request parameters? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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