In the CA MAT 12.0 installation guide there is:
The CA MAT Graphical User Interface (GUI) provides a graphical workstation-based interface for use with the CA MAT product. Mainframe support consists of a CA MAT Web Server to provide interface support to various CA MAT mainframe facilities. This server consists of three files that support running a job in z/OS UNIX System Services (USS). The CA MAT Web server requires 64-bit IBM Java 8.0.
It is not recommend that one tries to go ahead with the installation with a previous version of Java, like Java 7.1, because it does look mandatory to use Java 8.
Java 8 is required because it's the requirement for RAP.