Tomcat Invalid character found in the request target

While running an application in the tomcat server, sometimes we might get the following error.

org.apache.coyote.http11.Http11Processor.service Error parsing HTTP request header
 Note: further occurrences of HTTP request parsing errors will be logged at DEBUG level.
	java.lang.IllegalArgumentException: Invalid character found in the request target [/index.php?s=/Index/\think\app/invokefunction&function=call_user_func_array&vars[0]=md5&vars[1][]=HelloThinkPHP21 ]. The valid characters are defined in RFC 7230 and RFC 3986
		at org.apache.coyote.http11.Http11InputBuffer.parseRequestLine(Http11InputBuffer.java:509)
		at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:511)
		at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
		at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:831)
		at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1673)
		at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
		at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
		at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
		at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
		at java.lang.Thread.run(Thread.java:748)

First, make sure that the domain pointed to the server is working fine or not expired.

From tomcat, it is suggested that

The HTTP/1.1 specification requires that certain characters are %nn encoded when used in URI query strings. Unfortunately, many user agents including all the major browsers are not compliant with this specification and use these characters in unencoded form. To prevent Tomcat rejecting such requests, this attribute may be used to specify the additional characters to allow. If not specified, no additional characters will be allowed. The value may be any combination of the following characters: " < > [ \ ] ^ ` { | } . Any other characters present in the value will be ignored.

Tomcat increased its security and no longer allows raw square brackets or the above characters in the query string.

In order to resolve this issue use relaxedQueryChars attributes inside the server.xml file of tomcat server. Make sure to back up the server.xml file before we change it.

Open the server.xml file using vim or vi and type Shift + i

sudo vim /path_to_tomcat/conf/server.xml

Press Esc and hit :wq! and save the changes

Restart the tomcat server.