[[cleaning-up-resources-in-a-ui]]
Cleaning up resources in a UI
-----------------------------

Vaadin UIs that are open on the client side send a regular heartbeat
to the server to indicate they are still alive, even though there is no
ongoing user interaction. When the server does not receive a valid
heartbeat for a given UI, it will eventually remove that UI from the
session.

By default, heartbeats are sent every five minutes, and the server
closes an UI after three missed heartbeats. The heartbeat interval can
be customized by providing an init parameter named `heartbeatInterval`
in your web.xml, setting its value to the desired interval in seconds:

[source,xml]
....
<web-app>
    <context-param>
        <param-name>heartbeatInterval</param-name>
        <param-value>120</param-value>
    </context-param>

    <!-- Or per-servlet: -->
    <servlet>
        <!-- ... -->
        <init-param>
            <param-name>heartbeatInterval</param-name>
            <param-value>120</param-value>
        </init-param>
    </servlet>
</web-app>
....

To do custom cleanup in your UIs, `DetachListener`s can be registered:

[source,java]
....
public class MyUI extends UI {
    @Override
    protected void init(VaadinRequest request) {
        addDetachListener(new DetachListener() {

            @Override
            public void detach(DetachEvent event) {
                releaseSomeResources();
            }
        });
    }

    private void releaseSomeResources() {
        // ...
    }
....

Of course, your UI could also implement `DetachListener` itself:

[source,java]
....
public class MyUI extends UI implements DetachListener {
    protected void init(VaadinRequest request) {
        addDetachListener(this);
    }

    @Override
    public void detach(DetachEvent event) {
        // do cleanup
    }
}
....

If you'd like to share a listener instance between multiple UIs, the
current UI can be queried:

[source,java]
....
public void detach(DetachEvent event) {
    // do cleanup
    event.getConnector().getUI();

    // or equivalent:
    UI.getCurrent();
}
....

Because heartbeat requests are just like any other request from the
servlet container's viewpoint, each heartbeat extends the lifetime of
the underlying HttpSession. This means that as long as there is an open
UI, the session never expires even though there is no user interaction.
This is desirable in some situations, but in others it may be not.

You can control this behavior by setting an init parameter named
`closeIdleSessions` to `true`. When it is set to true (the default is
`false`), the session will be closed if no UI is active. Before the
session is closed, the detach methods will be called, and cleanup is
performed.

[source,xml]
....
<context-param> <!-- or init-param -->
    <param-name>closeIdleSessions</param-name>
    <param-value>true</param-value>
</context-param>
....