Chapter 4
CREOLE: the GATE Component Model [#]

The GATE architecture is based on components: reusable chunks of software with well-defined interfaces that may be deployed in a variety of contexts. The design of GATE is based on an analysis of previous work on infrastructure for LE, and of the typical types of software entities found in the fields of NLP and CL (see in particular chapters 4–6 of [Cunningham 00]). Our research suggested that a profitable way to support LE software development was an architecture that breaks down such programs into components of various types. Because LE practice varies very widely (it is, after all, predominantly a research field), the architecture must avoid restricting the sorts of components that developers can plug into the infrastructure. The GATE framework accomplishes this via an adapted version of the Java Beans component framework from Sun, as described in section 4.2.

GATE components may be implemented by a variety of programming languages and databases, but in each case they are represented to the system as a Java class. This class may do nothing other than call the underlying program, or provide an access layer to a database; on the other hand it may implement the whole component.

GATE components are one of three types:

• LanguageResources (LRs) represent entities such as lexicons, corpora or ontologies;
• ProcessingResources (PRs) represent entities that are primarily algorithmic, such as parsers, generators or ngram modellers;
• VisualResources (VRs) represent visualisation and editing components that participate in GUIs.

The distinction between language resources and processing resources is explored more fully in section C.1.1. Collectively, the set of resources integrated with GATE is known as CREOLE: a Collection of REusable Objects for Language Engineering.

In the rest of this chapter:

• Section 4.3 describes the lifecycle of GATE components;
• Section 4.4 describes how Processing Resources can be grouped into applications;
• Section 4.5 describes the relationship between Language Resources and their data stores;
• Section 4.6 summarises GATE’s set of built-in components;
• Section 4.7 describes how configuration data for Resource types is supplied to GATE.

4.1 The Web and CREOLE [#]

GATE allows resource implementations and Language Resource persistent data to be distributed over the Web, and uses Java annotations and XML for configuration of resources (and GATE itself).

Resource implementations are grouped together as ‘plugins’, stored at a URL (when the resources are in the local file system this can be a file:/ URL). When a plugin is loaded into GATE it looks for a configuration file called creole.xml relative to the plugin URL and uses the contents of this file to determine what resources this plugin declares and where to find the classes that implement the resource types (typically these classes are stored in a JAR file in the plugin directory). Configuration data for the resources may be stored directly in the creole.xml file, or it may be stored as Java annotations on the resource classes themselves; in either case GATE retrieves this configuration information and adds the resource definitions to the CREOLE register. When a user requests an instantiation of a resource, GATE creates an instance of the resource class in the virtual machine.

Language resource data can be stored in binary serialised form in the local file system.

4.2 The GATE Framework [#]

We can think of the GATE framework as a backplane into which users can plug CREOLE components. The user gives the system a list of URLs to search when it starts up, and components at those locations are loaded by the system.

The backplane performs these functions:

• component discovery, bootstrapping, loading and reloading;
• management and visualisation of native data structures for common information types;
• generalised data storage and process execution.

A set of components plus the framework is a deployment unit which can be embedded in another application.

At their most basic, all GATE resources are Java Beans, the Java platform’s model of software components. Beans are simply Java classes that obey certain interface conventions:

• beans must have no-argument constructors.
• beans have properties, defined by pairs of methods named by the convention setProp and getProp .

GATE uses Java Beans conventions to construct and configure resources at runtime, and defines interfaces that different component types must implement.

4.3 The Lifecycle of a CREOLE Resource [#]

CREOLE resources exhibit a variety of forms depending on the perspective they are viewed from. Their implementation is as a Java class plus an XML metadata file living at the same URL. When using GATE Developer, resources can be loaded and viewed via the resources tree (left pane) and the ‘create resource’ mechanism. When programming with GATE Embedded, they are Java objects that are obtained by making calls to GATE’s Factory class. These various incarnations are the phases of a CREOLE resource’s ‘lifecycle’. Depending on what sort of task you are using GATE for, you may use resources in any or all of these phases. For example, you may only be interested in getting a graphical view of what GATE’s ANNIE Information Extraction system (see Chapter 6) does; in this case you will use GATE Developer to load the ANNIE resources, and load a document, and create an ANNIE application and run it on the document. If, on the other hand, you want to create your own resources, or modify the Java code of an existing resource (as opposed to just modifying its grammar, for example), you will need to deal with all the lifecycle phases.

The various phases may be summarised as:

Creating a new resource from scratch (bootstrapping).

To create the binary image of a resource (a Java class in a JAR file), and the XML file that describes the resource to GATE, you need to create the appropriate .java file(s), compile them and package them as a .jar. GATE provides a bootstrap tool to start this process – see Section 7.10. Alternatively you can simply copy code from an existing resource.

Instantiating a resource in GATE Embedded.

To create a resource in your own Java code, use GATE’s Factory class (this takes care of parameterising the resource, restoring it from a database where appropriate, etc. etc.). Section 7.2 describes how to do this.

Loading a resource into GATE Developer.

To load a resource into GATE Developer, use the various ‘New ... resource’ options from the File menu and elsewhere. See Section 3.1.

Resource configuration and implementation.

GATE’s bootstrap tool will create an empty resource that does nothing. In order to achieve the behaviour you require, you’ll need to change the configuration of the resource (by editing the creole.xml file) and/or change the Java code that implements the resource. See section 4.7.

4.4 Processing Resources and Applications [#]

PRs can be combined into applications. Applications model a control strategy for the execution of PRs. In GATE, applications are called ‘controllers’ accordingly.

Currently only sequential, or pipeline, execution is supported. There are two main types of pipeline:

Simple pipelines

simply group a set of PRs together in order and execute them in turn. The implementing class is called SerialController.

Corpus pipelines

are specific for LanguageAnalysers – PRs that are applied to documents and corpora. A corpus pipeline opens each document in the corpus in turn, sets that document as a runtime parameter on each PR, runs all the PRs on the corpus, then closes the document. The implementing class is called SerialAnalyserController.

Conditional versions of these controllers are also available. These allow processing resources to be run conditionally on document features. See Section 3.7.1 for how to use these.

Controllers are themselves PRs – in particular a simple pipeline is a standard PR and a corpus pipeline is a LanguageAnalyser – so one pipeline can be nested in another. This is particularly useful with conditional controllers to group together a set of PRs that can all be turned on or off as a group.

There is also a real-time version of the corpus pipeline. When creating such a controller, a timeout parameter needs to be set which determines the maximum amount of time (in milliseconds) allowed for the processing of a document. Documents that take longer to process, are simply ignored and the execution moves to the next document after the timeout interval has lapsed.

All controllers have special handling for processing resources that implement the interface gate.creole.ControllerAwarePR. This interface provides methods that are called by the controller at the start and end of the whole application’s execution – for a corpus pipeline, this means before any document has been processed and after all documents in the corpus have been processed, which is useful for PRs that need to share data structures across the whole corpus, build aggregate statistics, etc. For full details, see the JavaDoc documentation for ControllerAwarePR.

4.5 Language Resources and Datastores [#]

Language Resources can be stored in Data Stores. Data Stores are an abstract model of disk-based persistence, which can be implemented by various types of storage mechanism. Here are the types implemented:

Serial Data Stores

are based on Java’s serialisation system, and store data directly into files and directories.

Lucene Data Stores

is a full-featured annotation indexing and retrieval system. It is provided as part of an extension of the Serial Data Stores. See Section 9 for more details.

4.6 Built-in CREOLE Resources [#]

GATE comes with various built-in components:

• Language Resources modelling Documents and Corpora, and various types of Annotation Schema – see Chapter 5.
• Processing Resources that are part of the ANNIE system – see Chapter 6.
• Gazetteers – see Chapter 13.
• Ontologies – see Chapter 14.
• Machine Learning resources – see Chapter 15.
• Alignment tools – see Chapter 16.
• Parsers and taggers – see Chapter 17.
• Other miscellaneous resources – see Chapter 19.

4.7 CREOLE Resource Configuration [#]

This section describes how to supply GATE with the configuration data it needs about a resource, such as what its parameters are, how to display it if it has a visualisation, etc. Several GATE resources can be grouped into a single plugin, which is a directory containing an XML configuration file called creole.xml. Configuration data for the plugin’s resources can be given in the creole.xml file or directly in the Java source file using Java 5 annotations.

A creole.xml file has a root element <CREOLE-DIRECTORY>, but the further contents of this element depend on the configuration style. The following three sections discuss the different styles – all-XML, all-annotations and a mixture of the two.

4.7.1 Configuration with XML [#]

To configure your resources in the creole.xml file, the <CREOLE-DIRECTORY> element should contain one <RESOURCE> element for each resource type in the plugin. The <RESOURCE> elements may optionally be contained within a <CREOLE> element (to allow a single creole.xml file to be built up by concatenating multiple separate files). For example:

Basic Resource-Level Data

Each resource must give a name, a Java class and the JAR file that it can be loaded from. The above example is taken from the Parser_Minipar plugin, and defines a single resource with a number of parameters.

The full list of valid elements under <RESOURCE> is as follows:

NAME

the name of the resource, as it will appear in the ‘New’ menu in GATE Developer. If omitted, defaults to the bare name of the resource class (without a package name).

CLASS

the fully qualified name of the Java class that implements this resource.

JAR

names JAR files required by this resource (paths are relative to the location of creole.xml). Typically this will be the JAR file containing the class named by the <CLASS> element, but additional <JAR> elements can be used to name third-party JAR files that the resource depends on.

COMMENT

a descriptive comment about the resource, which will appear as the tooltip when hovering over an instance of this resource in the resources tree in GATE Developer. If omitted, no comment is used.

HELPURL

a URL to a help document on the web for this resource. It is used in the help browser inside GATE Developer.

INTERFACE

the interface type implemented by this resource, for example new types of document would specify <INTERFACE>gate.Document</INTERFACE>.

ICON

the icon used to represent this resource in GATE Developer. This is a path inside the plugin’s JAR file, for example <ICON>/some/package/icon.png</ICON>. If the path specified does not start with a forward slash, it is assumed to name an icon from the GATE default set, which is located in gate.jar at gate/resources/img. If no icon is specified, a generic language resource or processing resource icon (as appropriate) is used.

PRIVATE

if present, this resource type is hidden in the GATE Developer GUI, i.e. it is not shown in the ‘New’ menus. This is useful for resource types that are intended to be created internally by other resources, or for resources that have parameters of a type that cannot be set in the GUI. <PRIVATE/> resources can still be created in Java code using the Factory.

AUTOINSTANCE (and HIDDEN-AUTOINSTANCE)

tells GATE to automatically create instances of this resource when the plugin is loaded. Any number of auto instances may be defined, GATE will create them all. Each <AUTOINSTANCE> element may optionally contain <PARAM NAME="..." VALUE="..." /> elements giving parameter values to use when creating the instance. Any parameters not specified explicitly will take their default values. Use <HIDDEN-AUTOINSTANCE> if you want the auto instances not to show up in GATE Developer – this is useful for things like document formats where there should only ever be a single instance in GATE and that instance should not be deleted.

For visual resources, a <GUI> element should also be provided. This takes a TYPE attribute, which can have the value LARGE or SMALL. LARGE means that the visual resource is a large viewer and should appear in the main part of the GATE Developer window on the right hand side, SMALL means the VR is a small viewer which appears in the space below the resources tree in the bottom left. The <GUI> element supports the following sub-elements:

RESOURCE_DISPLAYED

the type of GATE resource this VR can display. Any resource whose type is assignable to this type will be displayed with this viewer, so for example a VR that can display all types of document would specify gate.Document, whereas a VR that can only display the default GATE document implementation would specify gate.corpora.DocumentImpl.

MAIN_VIEWER

if present, GATE will consider this VR to be the ‘most important’ viewer for the given resource type, and will ensure that if several different viewers are all applicable to this resource, this viewer will be the one that is initially visible.

For annotation viewers, you should specify an <ANNOTATION_TYPE_DISPLAYED> element giving the annotation type that the viewer can display (e.g. Sentence).

Resource Parameters

Resources may also have parameters of various types. These resources, from the GATE distribution, illustrate the various types of parameters:

Parameters may be optional, and may have default values (and may have comments to describe their purpose, which is displayed by GATE Developer during interactive parameter setting).

Some PR parameters are execution time (RUNTIME), some are initialisation time. E.g. at execution time a doc is supplied to a language analyser; at initilisation time a grammar may be supplied to a language analyser.

The <PARAMETER> tag takes the following attributes:

NAME:

name of the JavaBean property that the parameter refers to, i.e. for a parameter named ‘someParam’ the class must have setSomeParam and getSomeParam methods.¹

DEFAULT:

default value (see below).

RUNTIME:

doesn’t need setting at initialisation time, but must be set before calling execute(). Only meaningfull for PRs

OPTIONAL:

not required

COMMENT:

for display purposes

ITEM_CLASS_NAME:

(only applies to parameters whose type is java.util.Collection or a type that implements or extends this) this specifies the type of elements the collection contains, so GATE can use the right type when parameters are set. If omitted, GATE will pass in the elements as Strings.

SUFFIXES:

(only applies to parameters of type java.net.URL) a semicolon-separated list of file suffixes that this parameter typically accepts, used as a filter in the file chooser provided by GATE Developer to select a local file as the parameter value.

It is possible for two or more parameters to be mutually exclusive (i.e. a user must specify one or the other but not both). In this case the <PARAMETER> elements should be grouped together under an <OR> element.

The type of the parameter is specified as the text of the <PARAMETER> element, and the type supplied must match the return type of the parameter’s get method. Any reference type (class, interface or enum) may be used as the parameter type, including other resource types – in this case GATE Developer will offer a list of the loaded instances of that resource as options for the parameter value. Primitive types (char, boolean, …) are not supported, instead you should use the corresponding wrapper type (java.lang.Character, java.lang.Boolean, …). If the getter returns a parameterized type (e.g. List<Integer>) you should just specify the raw type (java.util.List) here².

The DEFAULT string is converted to the appropriate type for the parameter - java.lang.String parameters use the value directly, primitive wrapper types e.g. java.lang.Integer use their respective valueOf methods, and other built-in Java types can have defaults specified provided they have a constructor taking a String.

The type java.net.URL is treated specially: if the default string is not an absolute URL (e.g. http://gate.ac.uk/) then it is treated as a path relative to the location of the creole.xml file. Thus a DEFAULT of ‘resources/main.jape’ in the file file:/opt/MyPlugin/creole.xml is treated as the absolute URL file:/opt/MyPlugin/resources/main.jape.

For Collection-valued parameters multiple values may be specified, separated by semicolons, e.g. ‘foo;bar;baz’; if the parameter’s type is an interface – Collection or one of its sub-interfaces (e.g. List) – a suitable concrete class (e.g. ArrayList, HashSet) will be chosen automatically for the default value.

For parameters of type gate.FeatureMap multiple name=value pairs can be specified, e.g. ‘kind=word;orth=upperInitial’. For enum-valued parameters the default string is taken as the name of the enum constant to use. Finally, if no DEFAULT attribute is specified, the default value is null.

4.7.2 Configuring Resources using Annotations [#]

As an alternative to the XML configuration style, GATE provides Java 5 annotation types to embed the configuration data directly in the Java source code. @CreoleResource is used to mark a class as a GATE resource, and parameter information is provided through annotations on the JavaBean set methods. At runtime these annotations are read and mapped into the equivalent entries in creole.xml before parsing. The metadata annotation types are all marked @Documented so the CREOLE configuration data will be visible in the generated JavaDoc documentation.

For more detailed information, see the JavaDoc documentation for gate.creole.metadata.

To use annotation-driven configuration a creole.xml file is still required but it need only contain the following:

This tells GATE to load myPlugin.jar and scan its contents looking for resource classes annotated with @CreoleResource. Other JAR files required by the plugin can be specified using other <JAR> elements without SCAN="true".

Basic Resource-Level Data

To mark a class as a CREOLE resource, simply use the @CreoleResource annotation (in the gate.creole.metadata package), for example:

The @CreoleResource annotation provides slots for all the values that can be specified under <RESOURCE> in creole.xml, except <CLASS> (inferred from the name of the annotated class) and <JAR> (taken to be the JAR containing the class):

name

(String) the name of the resource, as it will appear in the ‘New’ menu in GATE Developer. If omitted, defaults to the bare name of the resource class (without a package name). (XML equivalent <NAME>)

comment

(String) a descriptive comment about the resource, which will appear as the tooltip when hovering over an instance of this resource in the resources tree in GATE Developer. If omitted, no comment is used. (XML equivalent <COMMENT>)

helpURL

(String) a URL to a help document on the web for this resource. It is used in the help browser inside GATE Developer. (XML equivalent <HELPURL>)

isPrivate

(boolean) should this resource type be hidden from the GATE Developer GUI, so it does not appear in the ‘New’ menus? If omitted, defaults to false (i.e. not hidden). (XML equivalent <PRIVATE/>)

icon

(String) the icon to use to represent the resource in GATE Developer. If omitted, a generic language resource or processing resource icon is used. (XML equivalent <ICON>, see the description above for details)

interfaceName

(String) the interface type implemented by this resource, for example a new type of document would specify "gate.Document" here. (XML equivalent <INTERFACE>)

autoInstances

(array of @AutoInstance annotations) definitions for any instances of this resource that should be created automatically when the plugin is loaded. If omitted, no auto-instances are created by default. (XML equivalent, one or more <AUTOINSTANCE> and/or <HIDDEN-AUTOINSTANCE> elements, see the description above for details)

For visual resources only, the following elements are also available:

guiType

(GuiType enum) the type of GUI this resource defines. (XML equivalent <GUI TYPE="LARGE|SMALL">)

resourceDisplayed

(String) the class name of the resource type that this VR displays, e.g. "gate.Corpus". (XML equivalent <RESOURCE_DISPLAYED>)

mainViewer

(boolean) is this VR the ‘most important’ viewer for its displayed resource type? (XML equivalent <MAIN_VIEWER/>, see above for details)

For annotation viewers, you should specify an annotationTypeDisplayed element giving the annotation type that the viewer can display (e.g. Sentence).

Resource Parameters

Parameters are declared by placing annotations on their JavaBean set methods. To mark a setter method as a parameter, use the @CreoleParameter annotation, for example:

GATE will infer the parameter’s name from the name of the JavaBean property in the usual way (i.e. strip off the leading set and convert the following character to lower case, so in this example the name is abbrListUrl). The parameter name is not taken from the name of the method parameter. The parameter’s type is inferred from the type of the method parameter (java.net.URL in this case).

The annotation elements of @CreoleParameter correspond to the attributes of the <PARAMETER> tag in the XML configuration style:

comment

(String) an optional descriptive comment about the parameter. (XML equivalent COMMENT)

defaultValue

(String) the optional default value for this parameter. The value is specified as a string but is converted to the relevant type by GATE according to the conversions described in the previous section. Note that relative path default values for URL-valued parameters are still relative to the location of the creole.xml file, not the annotated class. (XML equivalent DEFAULT)

suffixes

(String) for URL-valued parameters, a semicolon-separated list of default file suffixes that this parameter accepts. (XML equivalent SUFFIXES)

collectionElementType

(Class) for Collection-valued parameters, the type of the elements in the collection. This can usually be inferred from the generic type information, for example public void setIndices(List<Integer> indices), but must be specified if the set method’s parameter has a raw (non-parameterized) type. (XML equivalent ITEM_CLASS_NAME)

Mutually-exclusive parameters (such as would be grouped in an <OR> in creole.xml) are handled by adding a disjunction="label" to the @CreoleParameter annotation – all parameters that share the same label are grouped in the same disjunction.

Optional and runtime parameters are marked using extra annotations, for example:

Inheritance

Unlike with pure XML configuration, when using annotations a resource will inherit any configuration data that was not explicitly specified from annotations on its parent class and on any interfaces it implements. Specifically, if you do not specify a comment, interfaceName, icon, annotationTypeDisplayed or the GUI-related elements (guiType and resourceDisplayed) on your @CreoleResource annotation then GATE will look up the class tree for other @CreoleResource annotations, first on the superclass, its superclass, etc., then at any implemented interfaces, and use the first value it finds. This is useful if you are defining a family of related resources that inherit from a common base class.

The resource name and the isPrivate and mainViewer flags are not inherited.

Parameter definitions are inherited in a similar way. This is one of the big advantages of annotation configuration over pure XML – if one resource class extends another then with pure XML configuration all the parent class’s parameter definitions must be duplicated in the subclass’s creole.xml definition. With annotations, parameters are inherited from the parent class (and its parent, etc.) as well as from any interfaces implemented. For example, the gate.LanguageAnalyser interface provides two parameter definitions via annotated set methods, for the corpus and document parameters. Any @CreoleResource annotated class that implements LanguageAnalyser, directly or indirectly, will get these parameters automatically.

Of course, there are some cases where this behaviour is not desirable, for example if a subclass calculates a value for a superclass parameter rather than having the user set it directly. In this case you can hide the parameter by overriding the set method in the subclass and using a marker annotation:

The overriding method will typically just call the superclass one, as its only purpose is to provide a place to put the @HiddenCreoleParameter annotation.

Alternatively, you may want to override some of the configuration for a parameter but inherit the rest from the superclass. Again, this is handled by trivially overriding the set method and re-annotating it:

Note that for backwards compatibility, data is only inherited from superclass annotations if the subclass is itself annotated with @CreoleResource. If the subclass is not annotated then GATE assumes that all its configuration is contained in creole.xml in the usual way.

4.7.3 Mixing the Configuration Styles [#]

It is possible and often useful to mix and match the XML and annotation-driven configuration styles. The rule is always that anything specified in the XML takes priority over the annotations. The following examples show what this allows.

Overriding Configuration for a Third-Party Resource

Suppose you have a plugin from some third party that uses annotation-driven configuration. You don’t have the source code but you would like to override the default value for one of the parameters of one of the plugin’s resources. You can do this in the creole.xml:

The default value for the listUrl parameter in the annotated class will be replaced by your value.

External AUTOINSTANCEs

For resources like document formats, where there should always and only be one instance in GATE at any time, it makes sense to put the auto-instance definitions in the @CreoleResource annotation. But if the automatically created instances are a convenience rather than a neccessity it may be better to define them in XML so other users can disable them without re-compiling the class:

Inheriting Parameters

If you would prefer to use XML configuration for your own resources, but would like to benefit from the parameter inheritance features of the annotation-driven approach, you can write a normal creole.xml file with all your configuration and just add a blank @CreoleResource annotation to your class. For example:

N.B. Without the @CreoleResource the parameters would not be inherited.