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JMRI Code: XML Persistance

JMRI uses XML for persisting internal structures, especially when storing the preferences and panel files.

XML persistance is done via some explicitly written code. Basically, certain classes register themselves with a instance of the "ConfigureManager". Normally, that will be the implementation that stores to and loads from XML files: jmri.configurexml.ConfigXmlManager. When it's time to store, the ConfigureXmlManager is told to do it. It goes through the registered objects and finds the persisting class responsible for storing the object. E.g. class a.b.Foo will have the class a.b.configurexml.FooXml located. If that class is found, it's told to store the Foo object, and it adds Xml content to a JDOM document to do that. If it's not located, an error message is issued.

On load, an XML file is read by the manager. Each element is examined for a "class" attribute. If found, that class is loaded and handed the element to process. Etc.

Although the basic structure is cleanly separated, the code with the *Xml classes tends to have a lot of replication and special case. To keep that all sane, we do a lot of unit and CI testing on it.

Example

A LightManager knows about Lights.
There are lots of concrete classes implementing the Light interface:

These have their own internal information, which is not always the same.

There are also multiple LightManager concrete classes to handle them:

Each type of manager is stored and loaded via a persistance class, who is found by looking the a class with "Xml" appended to the name, in a "configurexml" direct subpackage:

In the case of Light concrete classes, the code for persisting the managers directly stores and loads the individual lights. This is because (so far) a given manager only has one type of Light (e.g. LnLightManager only has to worry about LnLight). In cases where this is not true, e.g. SignalHeads which have multiple classes, there are persistance classes for the individual objects in addition to the manager.

Adding More Information to a Class

If you want to store more state information, find the persisting class and add code to it to create and read attributes or elements.
Perhaps the easiest way to do this is to create a sample panel file with the objects you want to store in it:

  <sensors class="jmri.jmrix.cmri.serial.configurexml.SerialSensorManagerXml" />
    <sensor systemName="CS3001" />
  </sensor>
  <sensors class="jmri.managers.configurexml.InternalSensorManagerXml" />
   <sensor systemName="IS21" />
  </sensors>
  <signalheads class="jmri.configurexml.AbstractSignalHeadManagerXml">
    <signalhead class="jmri.configurexml.DoubleTurnoutSignalHeadXml" systemName="IH1P">
      <turnout systemName="CT10" userName="1-bit pulsed green" />
      <turnout systemName="CT2" userName="1-bit pulsed red" />
    </signalhead>
  </signalheads>

Note the "class" attributes. They give the fully-qualified name of the class that can load or store that particular element. In the case of Sensors, we see there are two managers in use: One for C/MRI, and one for internal Sensors. For SignalHeads, there's only one manager, jmri.configurexml.AbstractSignalHeadManager persisted by jmri.configurexml.AbstractSignalHeadManager, but each particular SignalHead implementing class has it's own persisting class.

To e.g. add more data to a sensor object, the jmri.jmrix.cmri.serial.configurexml.SerialSensorManagerXml and jmri.managers.configurexml.InternalSensorManagerXml classes would have to be modified. This is where all the code to transfer to and from the stored form should go; don't add code in e.g. primary classes SerialSensorManager and InternalSensorManager to translate to or from the stored form. This keeps the main classes internal and flexible, allowing the persistance to be worked on (and tested and debugged!) separately.

Boolean values should be stored as the strings "true" and "false".

The getAttributeBooleanValue, getAttributeIntValue, and getAttributeIntegerValue methods provide a simple way of parsing input and handling errors.

Although much of the early code stored information in attributes, consider putting the data being stored in elements instead. This makes for simpler XML Schema and XSLT definitions, and the structured nature can be easier for humans to read.

If you do add new attributes or elements, don't forget to update the format definition, see below.

Note that in some cases, there's an inheritance relationship amoung the persisting classes that can help. For example, the LocoNet LnSensorManagerXml class inherits from jmri.configurexml.AbstractSensorManagerConfigXML, which does almost all the work of storing and loading sensors.

Handling Errors

If theres a parsing error, exception, etc, it should be reported via the ErrorHandler. This accumulates reports and presents them (in a almost-useful way) to the user when appropriate.

For an example of how to do that, see the getAttributeBooleanValue method.

Persisting enum values

The "EnumIO" tool built into AbstractXmlAdapter that reads and writes enums via the names of their elements.

You add a member in your configureXml load/store class: 

    static final EnumIO<MyEmum> enumMap = new EnumIoNamesNumbers<>(MyEnum);

Then to store your value, in this example from the myEnumValue variable, you pass the enum value through that map element in your store method: 
            element.setAttribute("name", "" + enumMap.outputFromEnum(myEnumValue));

Storing into an element is similar To restore the value on load: 
	myEnumValue = enumMap.inputFromAttribute(element.getAttribute("name"));

There are several variations available, please see the Javadoc. Briefly:

Persisting References to NamedBeans

Classes should, but don't always, hold references to NamedBeans via NamedBean handles.

If you're adding persistance for a class that doesn't do that, please update it before going further. That will save a lot of future trouble.

To store a NamedBeanHandle reference, just store the result of the getName() method of the NamedBeanHandle. That's the name the user refers to it by.

To load a reference, retrieve that name, look up the corresponding NamedBean (typically with get(String) method of the corresponding manager) and then create the NamedBeanHandle via the usual call: InstanceManager.getDefault(jmri.NamedBeanHandleManager.class).getNamedBeanHandle(name, thing)

Class Migration

Sometimes, classes need to be moved to another package as part of code maintenance. Since the fully-qualified class name, including package name, has been written to files, if we just move the class it will break reading of those files (in addition to breaking any user-written code that might refer to them). To handle this: There's also a service-oriented approach. For more on that, please see jmri.configurexml.ClassMigration that works in a similar way. For an example, see jmri.jmrix.pi.configurexml.RaspberryPiClassMigration

Schema Management

JMRI controls XML semantics using XML Schema.

For example, layout information is stored in XML files as a "layout-config" element, whose contents are then defined via a schema file. These files are kept in the xml/schema directory.