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?

I am not sure, but init() and destroy() are called only once, but for every request it has to go through doFilter.... this behaviour is same as servlet init, destroy or service methods. — AurA, Feb 12 '14 at 06:56

Luiggi Mendoza · Accepted Answer · 2014-02-12T07:20:45.503

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).

Not a singleton, but equivalent to "@ApplicationScoped" in CDI essentially. — Jonathan S. Fisher, Jun 04 '14 at 18:13

score 3 · Answer 2 · answered Apr 07 '17 at 12:58

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>

Are Java Filter Singleton

2 Answers2