Mailcap file search order:

The MailcapCommandMap looks in various places in the user's system for mailcap file entries. When requests are made to search for commands in the MailcapCommandMap, it searches mailcap files in the following order:

1. Programatically added entries to the MailcapCommandMap instance.
2. The file .mailcap in the user's home directory.
3. The file mailcap in the Java runtime.
4. The file or resources named META-INF/mailcap.
5. The file or resource named META-INF/mailcap.default (usually found only in the activation.jar file).

(The current implementation looks for the mailcap file in the Java runtime in the directory java.home/conf if it exists, and otherwise in the directory java.home/lib, where java.home is the value of the "java.home" System property. Note that the "conf" directory was introduced in JDK 9.)

Mailcap file format:

Mailcap files must conform to the mailcap file specification (RFC 1524, A User Agent Configuration Mechanism For Multimedia Mail Format Information). The file format consists of entries corresponding to particular MIME types. In general, the specification specifies applications for clients to use when they themselves cannot operate on the specified MIME type. The MailcapCommandMap extends this specification by using a parameter mechanism in mailcap files that allows JavaBeans(tm) components to be specified as corresponding to particular commands for a MIME type.

When a mailcap file is parsed, the MailcapCommandMap recognizes certain parameter signatures, specifically those parameter names that begin with x-java-. The MailcapCommandMap uses this signature to find command entries for inclusion into its registries. Parameter names with the form x-java-<name> are read by the MailcapCommandMap as identifying a command with the name name. When the name is content-handler the MailcapCommandMap recognizes the class signified by this parameter as a DataContentHandler. All other commands are handled generically regardless of command name. The command implementation is specified by a fully qualified class name of a JavaBean(tm) component. For example; a command for viewing some data can be specified as: x-java-view=com.foo.ViewBean.

When the command name is fallback-entry, the value of the command may be true or false. An entry for a MIME type that includes a parameter of x-java-fallback-entry=true defines fallback commands for that MIME type that will only be used if no non-fallback entry can be found. For example, an entry of the form text/*; ; x-java-fallback-entry=true; x-java-view=com.sun.TextViewer specifies a view command to be used for any text MIME type. This view command would only be used if a non-fallback view command for the MIME type could not be found.

MailcapCommandMap aware mailcap files have the following general form:

# Comments begin with a '#' and continue to the end of the line.
<mime type>; ; <parameter list>
# Where a parameter list consists of one or more parameters,
# where parameters look like: x-java-view=com.sun.TextViewer
# and a parameter list looks like:
text/plain; ; x-java-view=com.sun.TextViewer; x-java-edit=com.sun.TextEdit
# Note that mailcap entries that do not contain 'x-java' parameters
# and comply to RFC 1524 are simply ignored:
image/gif; /usr/dt/bin/sdtimage %s