# Schema Processing using JAXP

Applications can invoke schema processing using the APIs provided in JAXP 1.3. This makes Saxon interchangeable with other schema processors implementing this interface. There is full information on these APIs in the Java documentation. The two main mechanisms are the `Validator` class, and the `ValidatorHandler` class. Sample applications using these interfaces are provided in the `samples/java` directory. Saxon also supplies the class `com.saxonica.jaxp.ValidatingReader`, which implements the SAX2 `XMLReader` interface, allowing it to be used as a schema-validating XML parser.

The main steps are:

1. Create a `SchemaFactory`, by calling `SchemaFactory.getInstance()` with the argument `"http://www.w3.org/2001/XMLSchema"`, and with the Java system properties set up to ensure that Saxon is loaded as the chosen schema processor. Saxon will normally be loaded as the default schema processor if Saxon-EE is present on the classpath, but to make absolutely sure, set the system property `javax.xml.validation.SchemaFactory:http://www.w3.org/2001/XMLSchema` to the value `com.saxonica.jaxp.SchemaFactoryImpl`. Note that if you set this property using a property file, colons in the property name must be escaped as "\:".

2. Process a schema document by calling one of the several `newSchema` methods on the returned `SchemaFactory`.

3. Create either a `Validator` or a `ValidatorHandler` from this returned `Schema`.

4. Use the `Validator` or `ValidatorHandler` to process one or more source documents.

Note that additional schemas referenced from the `xsi:schemaLocation` attributes within the source documents will be loaded as necessary. A target namespace is ignored if there is already a loaded schema for that namespace; Saxon makes no attempt to load multiple schemas for the same namespace and check them for consistency.

Although the API is defined in such a way that a `Validator` or `ValidatorHandler` is created for a particular `Schema`, in the Saxon implementation the schema components that are available to the validator are not only the components within that schema, but all the components that form part of any schema registered with the `Configuration`.

Another way to control validation from a Java application is to run a JAXP identity transformation, having first set the option to perform schema validation. The following code (from the sample application `QuickValidator.java`) illustrates this:

    try {
        System.setProperty(
                "javax.xml.transform.TransformerFactory",
                "com.saxonica.SchemaAwareTransformerFactory");
        TransformerFactory factory =
                TransformerFactory.newInstance();
        factory.setAttribute(FeatureKeys.SCHEMA_VALIDATION,
                new Integer(Validation.STRICT));
        Transformer trans = factory.newTransformer();
        StreamSource source =
                new StreamSource(new File(args[0]).toURI().toString());
        SAXResult sink =
                new SAXResult(new DefaultHandler());
        trans.transform(source, sink);
    } catch (TransformerException err) {
        System.err.println("Validation failed");
    }

If you set an `ErrorListener` on the `TransformerFactory`, then you can control the way that error messages are output.

If you want to validate against a schema without hard-coding the URI of the schema into the source document, you can do this by pre-loading the schema into the `TransformerFactory`. This extended example (again from the sample application `QuickValidator.java`) illustrates this:

    try {
        System.setProperty(
                "javax.xml.transform.TransformerFactory",
                "com.saxonica.SchemaAwareTransformerFactory");
        TransformerFactory factory =
                TransformerFactory.newInstance();
        factory.setAttribute(FeatureKeys.SCHEMA_VALIDATION,
                new Integer(Validation.STRICT));
        if (args.length > 1) {
            StreamSource schema =
                    new StreamSource(new File(args[1]).toURI().toString());
            ((SchemaAwareTransformerFactory)factory).addSchema(schema);
        }
        Transformer trans = factory.newTransformer();
        StreamSource source =
                new StreamSource(new File(args[0]).toURI().toString());
        SAXResult sink =
                new SAXResult(new DefaultHandler());
        trans.transform(source, sink);
    } catch (TransformerException err) {
        System.err.println("Validation failed");
    }

You can preload as many schemas as you like using the `addSchema` method. Such schemas are parsed, validated, and compiled once, and can be used as often as you like for validating multiple source documents. You cannot unload a schema once it has been loaded. If you want to remove or replace a schema, start afresh with a new `TransformerFactory`.

Behind the scenes, the `TransformerFactory` uses a `Configuration` object to hold all the configuration information. The basic Saxon product uses the class `net.sf.saxon.TransformerFactoryImpl` for the `TransformerFactory`, and `net.sf.saxon.Configuration` for the underlying configuration information. The schema-aware product subclasses these with `com.saxonica.config.SchemaAwareTransformerFactory` and `com.saxonica.config.EnterpriseConfiguration` respectively. You can get hold of the `Configuration` object by casting the `TransformerFactory` to a Saxon `TransformerFactorImpl` and calling the `getConfiguration()` method. This gives you more precise control, for example it allows you to retrieve the `Schema` object containing the schema components for a given target namespace, and to inspect the compiled schema to establish its properties. See the JavaDoc documentation for further details.

Saxon currently implements its own API for access to the schema components. This API should be regarded as temporary. In the longer term, it is possible that Saxon will offer an API for schema access that has been proposed in a member submission to W3C.

The programming approach outlined above, of using an identity transformer, is suitable for a wide class of applications. For example, it enables you to insert a validation step into a SAX-based pipeline. However, for finer control, there are lower-level interfaces available in Saxon that you can also use. See for example the JavaDoc for the `EnterpriseConfiguration` class, which includes methods such as `getElementValidator`. This constructs a `Receiver` which acts as a validating XML event filter. This can be inserted into a pipeline of `Receiver`s. Saxon also provides classes to bridge between SAX events and `Receiver` events: `ReceivingContentHandler` and `ContentHandlerProxy` respectively.

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