Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"

Question 1

Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"

html forms jsp servlets http-status-code-404

pongahead · Jul 31, 2012 · Viewed 129.6k times · Source

Answer

Answer

Put servlet class in a `package`

First of all, put the servlet class in a Java package. You should always put publicly reuseable Java classes in a package, otherwise they are invisible to classes which are in a package, such as the server itself. This way you eliminiate potential environment-specific problems. Packageless servlets work only in specific Tomcat+JDK combinations and this should never be relied upon.

In case of a "plain" IDE project, the class needs to be placed in its package structure inside "Java Resources" folder and thus not "WebContent", this is for web files such as JSP. Below is an example of the folder structure of a default Eclipse Dynamic Web Project as seen in Navigator view:

EclipseProjectName
 |-- src
 |    `-- com
 |         `-- example
 |              `-- YourServlet.java
 |-- WebContent
 |    |-- WEB-INF
 |    |    `-- web.xml
 |    `-- jsps
 |         `-- page.jsp
 :

In case of a Maven project, the class needs to be placed in its package structure inside main/java and thus not e.g. main/resources, this is for non-class files. Below is an example of the folder structure of a default Maven webapp project as seen in Eclipse's Navigator view:

MavenProjectName
 |-- src
 |    `-- main
 |         |-- java
 |         |    `-- com
 |         |         `-- example
 |         |              `-- YourServlet.java
 |         |-- resources
 |         `-- webapp
 |              |-- WEB-INF
 |              |    `-- web.xml
 |              `-- jsps
 |                   `-- page.jsp
 :

Note that the /jsps subfolder is not strictly necessary. You can even do without it and put the JSP file directly in webcontent/webapp root, but I'm just taking over this from your question.

Set servlet URL in `url-pattern`

The servlet URL is specified as the "URL pattern" of the servlet mapping. It's absolutely not per definition the classname/filename of the servlet class. The URL pattern is to be specified as value of @WebServlet annotation.

package com.example; // Use a package!

@WebServlet("/servlet") // This is the URL of the servlet.
public class YourServlet extends HttpServlet { // Must be public and extend HttpServlet.
    // ...
}

In case you want to support path parameters like /servlet/foo/bar, then use an URL pattern of /servlet/* instead. See also Servlet and path parameters like /xyz/{value}/test, how to map in web.xml?

`@WebServlet` works only on Servlet 3.0 or newer

In order to use @WebServlet, you only need to make sure that your web.xml file, if any (it's optional since Servlet 3.0), is declared conform Servlet 3.0+ version and thus not conform e.g. 2.5 version or lower. Below is a Servlet 4.0 compatible one (which matches Tomcat 9+, WildFly 11+, Payara 5+, etc).

<?xml version="1.0" encoding="UTF-8"?>
<web-app
    xmlns="http://xmlns.jcp.org/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
    version="4.0"
>
    <!-- Config here. -->
</web-app>

Or, in case you're not on Servlet 3.0+ yet (e.g. Tomcat 6 or older), then remove the @WebServlet annotation.

package com.example;

public class YourServlet extends HttpServlet {
    // ...
}

And register the servlet instead in web.xml like this:

<servlet>
    <servlet-name>yourServlet</servlet-name>
    <servlet-class>com.example.YourServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>yourServlet</servlet-name>
    <url-pattern>/servlet</url-pattern>  <!-- This is the URL of the servlet. -->
</servlet-mapping>

Note thus that you should not use both ways. Use either annotation based configuarion or XML based configuration. When you have both, then XML based configuration will override annotation based configuration.

Verifying the build/deployment

In case you're using a build tool such as Eclipse and/or Maven, then you need to make absolutely sure that the compiled servlet class file resides in its package structure in /WEB-INF/classes folder of the produced WAR file. In case of package com.example; public class YourServlet, it must be located in /WEB-INF/classes/com/example/YourServlet.class. Otherwise you will face in case of @WebServlet also a 404 error, or in case of <servlet> a HTTP 500 error like below:

HTTP Status 500

Error instantiating servlet class com.example.YourServlet

And find in the server log a java.lang.ClassNotFoundException: com.example.YourServlet, followed by a java.lang.NoClassDefFoundError: com.example.YourServlet, in turn followed by javax.servlet.ServletException: Error instantiating servlet class com.example.YourServlet.

An easy way to verify if the servlet is correctly compiled and placed in classpath is to let the build tool produce a WAR file (e.g. rightclick project, Export > WAR file in Eclipse) and then inspect its contents with a ZIP tool. If the servlet class is missing in /WEB-INF/classes, or if the export causes an error, then the project is badly configured or some IDE/project configuration defaults have been mistakenly reverted (e.g. Project > Build Automatically has been disabled in Eclipse).

You also need to make sure that the project icon has no red cross indicating a build error. You can find the exact error in Problems view (Window > Show View > Other...). Usually the error message is fine Googlable. In case you have no clue, best is to restart from scratch and do not touch any IDE/project configuration defaults. In case you're using Eclipse, you can find instructions in How do I import the javax.servlet API in my Eclipse project?

Testing the servlet individually

Provided that the server runs on localhost:8080, and that the WAR is successfully deployed on a context path of /contextname (which defaults to the IDE project name, case sensitive!), and the servlet hasn't failed its initialization (read server logs for any deploy/servlet success/fail messages and the actual context path and servlet mapping), then a servlet with URL pattern of /servlet is available at http://localhost:8080/contextname/servlet.

You can just enter it straight in browser's address bar to test it invidivually. If its doGet() is properly overriden and implemented, then you will see its output in browser. Or if you don't have any doGet() or if it incorrectly calls super.doGet(), then a "HTTP 405: HTTP method GET is not supported by this URL" error will be shown (which is still better than a 404 as a 405 is evidence that the servlet itself is actually found).

Overriding service() is a bad practice, unless you're reinventing a MVC framework — which is very unlikely if you're just starting out with servlets and are clueless as to the problem described in the current question ;) See also Design Patterns web based applications.

Regardless, if the servlet already returns 404 when tested invidivually, then it's entirely pointless to try with a HTML form instead. Logically, it's therefore also entirely pointless to include any HTML form in questions about 404 errors from a servlet.

Referencing the servlet URL from HTML

Once you've verified that the servlet works fine when invoked individually, then you can advance to HTML. As to your concrete problem with the HTML form, the <form action> value needs to be a valid URL. The same applies to <a href>. You need to understand how absolute/relative URLs work. You know, an URL is a web address as you can enter/see in the webbrowser's address bar. If you're specifying a relative URL as form action, i.e. without the http:// scheme, then it becomes relative to the current URL as you see in your webbrowser's address bar. It's thus absolutely not relative to the JSP/HTML file location in server's WAR folder structure as many starters seem to think.

So, assuming that the JSP page with the HTML form is opened by http://localhost:8080/contextname/jsps/page.jsp, and you need to submit to a servlet located in http://localhost:8080/contextname/servlet, here are several cases (note that you can safely substitute <form action> with <a href> here):

Form action submits to an URL with a leading slash.
```
  <form action="/servlet">
```
The leading slash / makes the URL relative to the domain, thus the form will submit to
```
  http://localhost:8080/servlet
```
But this will likely result in a 404 as it's in the wrong context.
Form action submits to an URL without a leading slash.
```
  <form action="servlet">
```
This makes the URL relative to the current folder of the current URL, thus the form will submit to
```
  http://localhost:8080/contextname/jsps/servlet
```
But this will likely result in a 404 as it's in the wrong folder.
Form action submits to an URL which goes one folder up.
```
  <form action="../servlet">
```
This will go one folder up (exactly like as in local disk file system paths!), thus the form will submit to
```
  http://localhost:8080/contextname/servlet
```
This one must work!
The canonical approach, however, is to make the URL domain-relative so that you don't need to fix the URLs once again when you happen to move the JSP files around into another folder.
```
  <form action="${pageContext.request.contextPath}/servlet">
```
This will generate
```
  <form action="/contextname/servlet">
```
Which will thus always submit to the right URL.

Use straight quotes in HTML

You need to make absolutely sure you're using straight quotes in HTML attributes like action="..." or action='...' and thus not curly quotes like action=”...” or action=’...’. Curly quotes are not supported in HTML and they will simply become part of the value. Watch out when copy-pasting code snippets from blogs! Some blog engines, notably Wordpress, are known to by default use so-called "smart quotes" which thus also corrupts the quotes in code snippets this way. On the other hand, instead of copy-pasting code, try simply typing over the code yourself. Additional advantage of actually getting the code through your brain and fingers is that it will make you to remember and understand the code much better in long term and also make you a better developer.

Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"

HTTP Status 404 — /servlet

HTTP Status 404 — Not Found

Answer

Put servlet class in a `package`

Set servlet URL in `url-pattern`

`@WebServlet` works only on Servlet 3.0 or newer

Verifying the build/deployment

Testing the servlet individually

Referencing the servlet URL from HTML

Use straight quotes in HTML

See also:

Other cases of HTTP Status 404 error:

Servlet returns "HTTP Status 404 The requested resource (/servlet) is not available"

HTTP Status 404 — /servlet

HTTP Status 404 — Not Found

Answer

Put servlet class in a package

Set servlet URL in url-pattern

@WebServlet works only on Servlet 3.0 or newer

Verifying the build/deployment

Testing the servlet individually

Referencing the servlet URL from HTML

Use straight quotes in HTML

See also:

Other cases of HTTP Status 404 error:

Related questions

Put servlet class in a `package`

Set servlet URL in `url-pattern`

`@WebServlet` works only on Servlet 3.0 or newer