WebSocket How-To
Table of Contents
Overview
Tomcat provides support for WebSocket as defined by RFC 6455.
Application development
Tomcat implements the Jakarta WebSocket 2.1 API defined by the Jakarta WebSocket project.
There are several example applications that demonstrate how the WebSocket API can be used. You will need to look at both the client side HTML and the server side code.
Tomcat WebSocket specific configuration
Tomcat provides a number of Tomcat specific configuration options for WebSocket. It is anticipated that these will be absorbed into the WebSocket specification over time.
The write timeout used when sending WebSocket messages in blocking mode
defaults to 20000 milliseconds (20 seconds). This may be changed by setting
the property org.apache.tomcat.websocket.BLOCKING_SEND_TIMEOUT
in the user properties collection attached to the WebSocket session. The
value assigned to this property should be a Long and represents
the timeout to use in milliseconds. For an infinite timeout, use
-1.
In addition to the Session.setMaxIdleTimeout(long) method which
is part of the Jakarta WebSocket API, Tomcat provides greater control of the
timing out the session due to lack of activity. Setting the property
org.apache.tomcat.websocket.READ_IDLE_TIMEOUT_MS in the user
properties collection attached to the WebSocket session will trigger a
session timeout if no WebSocket message is received for the specified number
of milliseconds. Setting the property
org.apache.tomcat.websocket.WRITE_IDLE_TIMEOUT_MS will trigger a
session timeout if no WebSocket message is sent for the specified number of
milliseconds. These can be used separately or together, with or without
Session.setMaxIdleTimeout(long). If the associated property is
not specified, the read and/or write idle timeout will be applied.
If the application does not define a MessageHandler.Partial for
incoming binary messages, any incoming binary messages must be buffered so
the entire message can be delivered in a single call to the registered
MessageHandler.Whole for binary messages. The default buffer
size for binary messages is 8192 bytes. This may be changed for a web
application by setting the servlet context initialization parameter
org.apache.tomcat.websocket.binaryBufferSize to the desired
value in bytes.
If the application does not define a MessageHandler.Partial for
incoming text messages, any incoming text messages must be buffered so the
entire message can be delivered in a single call to the registered
MessageHandler.Whole for text messages. The default buffer size
for text messages is 8192 bytes. This may be changed for a web application by
setting the servlet context initialization parameter
org.apache.tomcat.websocket.textBufferSize to the desired value
in bytes.
The Java WebSocket specification 1.0 does not permit programmatic deployment
after the first endpoint has started a WebSocket handshake. By default,
Tomcat continues to permit additional programmatic deployment. This
behavior is controlled by the
org.apache.tomcat.websocket.noAddAfterHandshake servlet context
initialization parameter. The default may be changed by setting the
org.apache.tomcat.websocket.STRICT_SPEC_COMPLIANCE system
property to true but any explicit setting on the servlet context
will always take priority.
When using the WebSocket client to connect to server endpoints, the timeout
for IO operations while establishing the connection is controlled by the
userProperties of the provided
jakarta.websocket.ClientEndpointConfig. The property is
org.apache.tomcat.websocket.IO_TIMEOUT_MS and is the
timeout as a String in milliseconds. The default is 5000 (5
seconds).
When using the WebSocket client to connect to secure server endpoints, the
client SSL configuration should be configured via
jakarta.websocket.ClientEndpointConfig.getSSLContext(). Tomcat
10.1.x still supports the pre-WebSocket 2.1 configuration method where TLS
configuration was via the userProperties of the provided
jakarta.websocket.ClientEndpointConfig. However, this approach
is deprecated and will be removed in Tomcat 11. The following user properties
are supported:
org.apache.tomcat.websocket.SSL_CONTEXTorg.apache.tomcat.websocket.SSL_PROTOCOLSorg.apache.tomcat.websocket.SSL_TRUSTSTOREorg.apache.tomcat.websocket.SSL_TRUSTSTORE_PWD
The default truststore password is changeit.
If the org.apache.tomcat.websocket.SSL_CONTEXT property is
set then the org.apache.tomcat.websocket.SSL_TRUSTSTORE and
org.apache.tomcat.websocket.SSL_TRUSTSTORE_PWD properties
will be ignored.
For secure server end points, host name verification is enabled by default.
To bypass this verification (not recommended), it is necessary to provide a
custom SSLContext via the
org.apache.tomcat.websocket.SSL_CONTEXT user property. The
custom SSLContext must be configured with a custom
TrustManager that extends
javax.net.ssl.X509ExtendedTrustManager. The desired verification
(or lack of verification) can then be controlled by appropriate
implementations of the individual abstract methods.
When using the WebSocket client to connect to server endpoints, the number of
HTTP redirects that the client will follow is controlled by the
userProperties of the provided
jakarta.websocket.ClientEndpointConfig. The property is