Are Java Filter Singleton

Question

I am implementing a Java enterprise application and declared a Filter for every request, so how does the server tracks this request, do it create a new filter object for every request, or their is only one filter handling all request, in other words are java web filter singletone?

Answer 1

First, let's review the definition of Singleton Pattern (emphasis mine):

In software engineering, the singleton pattern is a design pattern that restricts the instantiation of a class to one object.

When you declare a class that implements the Filter interface, it needs a public constructor (usually the default constructor) so the application server could instantiate it. Thus, by doing this, the Filter is not a singleton.

Note that the application server will maintain a single instance per application context e.g. per a deployed web application, but this is not the same as having a singleton. Why? Because you or another programmer can carelessly create an instance of this class (even if it doesn't make use of the instance).

Answer 2

The answer is depended on how you define it in web.xml.

For example, this fragment of web.xml, create one object of Filter1

    <filter>
        <filter-name>Filter1</filter-name>
        <filter-class>com.surasin.test.Filter1</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>Filter1</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

But this framgemet of web.xml, create two objects of Filter1

    <filter>
        <filter-name>Filter1</filter-name>
        <filter-class>com.surasin.test.Filter1</filter-class>
        <init-param>
           <param-name>my-param</param-name>
           <param-value>my-param-value</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>Filter1</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
    <filter>
        <filter-name>Filter1</filter-name>
        <filter-class>com.surasin.test.Filter1</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>Filter1</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>