In Java, Model 1 and Model 2 are two types of web application architectures used to build Java web applications.

Model 1 Architecture:

The Model 1 architecture is a simple architecture that doesn't follow any specific pattern. In this architecture, the Servlet acts as a controller as well as a view. It is responsible for handling the client's request and providing a response by querying the database, manipulating data, and returning the results to the client. This architecture is also called the page-centric architecture, as each JSP or HTML page has its own servlet, which is responsible for handling the request and generating the response.

The following diagram shows the Model 1 architecture:

+---------+      +-------+
                  | Browser |<---->|  JSP  |
                  +---------+      +-------+
                       ^               |
                       |               |
                +-------------+  +--------+
                | Controller  |  |  Model |
                |   Servlet   |  | Classes|
                +-------------+  +--------+

Model 2 Architecture:

The Model 2 architecture is a widely used architecture that separates the application into three layers: Model, View, and Controller. In this architecture, the Servlet acts as a controller and delegates the request to the Model (business logic) to perform the required operations. The Model then returns the results to the controller, which, in turn, forwards it to the View (JSP or HTML) for rendering. This architecture provides a clear separation of concerns and makes the application more maintainable, reusable, and scalable.

The following diagram shows the Model 2 architecture:

+---------+      +--------+
                      | Browser |<---->|  View  |
                      +---------+      +--------+
                            ^               |
                            |               |
                     +-------------+  +--------+
                     | Controller  |  |  Model |
                     |   Servlet   |  | Classes|
                     +-------------+  +--------+

In Model 2 architecture, the View layer is responsible for presenting the data to the user in a user-friendly format, and the Model layer is responsible for business logic, data storage, and data retrieval. The Controller layer is responsible for handling the client's request, controlling the flow of the application, and delegating the request to the appropriate Model and View components.