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Chapter 4
CREOLE: the GATE Component Model [#]

…Noam Chomsky’s answer in Secrets, Lies and Democracy (David Barsamian 1994; Odonian) to ‘What do you think about the Internet?’

‘I think that there are good things about it, but there are also aspects of it that concern and worry me. This is an intuitive response – I can’t prove it – but my feeling is that, since people aren’t Martians or robots, direct face-to-face contact is an extremely important part of human life. It helps develop self-understanding and the growth of a healthy personality.

‘You just have a different relationship to somebody when you’re looking at them than you do when you’re punching away at a keyboard and some symbols come back. I suspect that extending that form of abstract and remote relationship, instead of direct, personal contact, is going to have unpleasant effects on what people are like. It will diminish their humanity, I think.’

Chomsky, quoted at http://philip.greenspun.com/wtr/dead-trees/53015.

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:

The distinction between language resources and processing resources is explored more fully in section D.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:

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:

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:

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 7.12. 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 3.8.2 for how to use these. If more flexibility is required, the Groovy plugin provides a scriptable controller (see section 7.17.3) whose execution strategy is specified using the Groovy programming language.

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 Datastores. Datastores are an abstract model of disk-based persistence, which can be implemented by various types of storage mechanism. Here are the types implemented:

Serial Datastores
are based on Java’s serialisation system, and store data directly into files and directories.
Lucene Datastores
is a full-featured annotation indexing and retrieval system. It is provided as part of an extension of the Serial Datastores. See Section 9 for more details.

4.6 Built-in CREOLE Resources [#]

GATE comes with various built-in components:

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 annotations.

A creole.xml file has a root element <CREOLE-DIRECTORY>. Traditionally this element didn’t contain any attributes, but with the introduction of installable plugins (see Sections 3.6 and 12.3.5) the following attributes can now be provided.

ID:
A string that uniquely identifies this plugin. This should be formatted in a similar way to fully specified Java class names. The class portion (i.e. everything after the last dot) will be used as the name of the plugin in the GUI. For example, the obsolete RASP plugin could have the ID gate.obsolete.RASP. Note that unlike Java class names the plugin name can contain spaces for the purpose of presentation.
VERSION:
The version number of the plugin. For example, 3, 3.1, 3.11, 3.12-SNAPSHOT etc.
DESCRIPTION:
A short description of the resources provided by the plugin. Note that there is really only space for a single sentence in the GUI.
HELPURL:
The URL of a web page giving more details about this plugin.
GATE-MIN:
The earliest version of GATE that this plugin is compatible with. This should be in the same format as the version shown in the GATE titlebar, i.e. 6.1 or 6.2-SNAPSHOT. Do not include the build number information.
GATE-MAX:
The last version of GATE which the plugin is compatible with. This should be in the same format as GATE-MIN.

Currently all these attributes are optional, unless you intend to make the plugin available through a plugin repository (see Section 12.3.5), in which case the ID and VERSION attributes must be provided. We would, however, suggest that developers start to add these attributes to all the plugins they develop as the information is likely to be used in more places throughout GATE developer and embeded in the future.

Child elements of the <CREOLE-DIRECTORY> 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:

<CREOLE-DIRECTORY>  
 
<CREOLE>  
  <RESOURCE>  
    <NAME>Minipar Wrapper</NAME>  
    <JAR>MiniparWrapper.jar</JAR>  
    <CLASS>minipar.Minipar</CLASS>  
    <COMMENT>MiniPar is a shallow parser. It determines the  
    dependency relationships between the words of a sentence.</COMMENT>  
    <HELPURL>http://gate.ac.uk/cgi-bin/userguide/sec:parsers:minipar</HELPURL>  
    <PARAMETER NAME="document"  
  RUNTIME="true"  
  COMMENT="document to process">gate.Document</PARAMETER>  
    <PARAMETER NAME="miniparDataDir"  
        RUNTIME="true"  
        COMMENT="location of the Minipar data directory">  
        java.net.URL  
    </PARAMETER>  
    <PARAMETER NAME="miniparBinary"  
        RUNTIME="true"  
        COMMENT="Name of the Minipar command file">  
        java.net.URL  
    </PARAMETER>  
    <PARAMETER NAME="annotationInputSetName"  
        RUNTIME="true"  
        OPTIONAL="true"  
        COMMENT="Name of the input Source">  
        java.lang.String  
    </PARAMETER>  
    <PARAMETER NAME="annotationOutputSetName"  
        RUNTIME="true"  
        OPTIONAL="true"  
        COMMENT="Name of the output AnnotationSetName">  
        java.lang.String  
    </PARAMETER>  
    <PARAMETER NAME="annotationTypeName"  
        RUNTIME="false"  
        DEFAULT="DepTreeNode"  
        COMMENT="Annotations to store with this type">  
        java.lang.String  
    </PARAMETER>  
  </RESOURCE>  
</CREOLE>  
</CREOLE-DIRECTORY>

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.
TOOL
if present, this resource type is considered to be a “tool”. Tools can contribute items to the Tools menu in GATE Developer.

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:

<RESOURCE>  
  <NAME>GATE document</NAME>  
  <CLASS>gate.corpora.DocumentImpl</CLASS>  
  <INTERFACE>gate.Document</INTERFACE>  
  <COMMENT>GATE transient document</COMMENT>  
  <OR>  
    <PARAMETER NAME="sourceUrl"  
      SUFFIXES="txt;text;xml;xhtm;xhtml;html;htm;sgml;sgm;mail;email;eml;rtf"  
      COMMENT="Source URL">java.net.URL</PARAMETER>  
    <PARAMETER NAME="stringContent"  
      COMMENT="The content of the document">java.lang.String</PARAMETER>  
  </OR>  
  <PARAMETER  
    COMMENT="Should the document read the original markup"  
    NAME="markupAware" DEFAULT="true">java.lang.Boolean</PARAMETER>  
  <PARAMETER NAME="encoding" OPTIONAL="true"  
    COMMENT="Encoding" DEFAULT="">java.lang.String</PARAMETER>  
  <PARAMETER NAME="sourceUrlStartOffset"  
    COMMENT="Start offset for documents based on ranges"  
    OPTIONAL="true">java.lang.Long</PARAMETER>  
  <PARAMETER NAME="sourceUrlEndOffset"  
    COMMENT="End offset for documents based on ranges"  
    OPTIONAL="true">java.lang.Long</PARAMETER>  
  <PARAMETER NAME="preserveOriginalContent"  
    COMMENT="Should the document preserve the original content"  
    DEFAULT="false">java.lang.Boolean</PARAMETER>  
  <PARAMETER NAME="collectRepositioningInfo"  
    COMMENT="Should the document collect repositioning information"  
    DEFAULT="false">java.lang.Boolean</PARAMETER>  
  <ICON>lr.gif</ICON>  
</RESOURCE>

<RESOURCE>  
  <NAME>Document Reset PR</NAME>  
  <CLASS>gate.creole.annotdelete.AnnotationDeletePR</CLASS>  
  <COMMENT>Document cleaner</COMMENT>  
  <PARAMETER NAME="document" RUNTIME="true">gate.Document</PARAMETER>  
  <PARAMETER NAME="annotationTypes" RUNTIME="true"  
    OPTIONAL="true">java.util.ArrayList</PARAMETER>  
</RESOURCE>

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 initialisation 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 meaningful 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 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 for a plugin a creole.xml file is still required but it need only contain the following:

<CREOLE-DIRECTORY>  
  <JAR SCAN="true">myPlugin.jar</JAR>  
  <JAR>lib/thirdPartyLib.jar</JAR>  
</CREOLE-DIRECTORY>

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".

In a GATE Embedded application it is possible to register a single @CreoleResource annotated class without using a creole.xml file by calling

Gate.getCreoleRegister().registerComponent(MyResource.class);

GATE will extract the configuration from the annotations on the class and make it available for use as if it had been defined in a plugin.

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:

1import gate.creole.AbstractLanguageAnalyser; 
2import gate.creole.metadata.*; 
3 
4@CreoleResource(name = "GATE Tokeniser", 
5                comment = "Splits text into tokens and spaces") 
6public class Tokeniser extends AbstractLanguageAnalyser { 
7  ...

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)
tool
(boolean) is this resource type a tool?

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:

  @CreoleParameter(comment = "The location of the list of abbreviations")  
  public void setAbbrListUrl(URL listUrl) {  
    ...

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 class3. (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" and priority=n to the @CreoleParameter annotation – all parameters that share the same label are grouped in the same disjunction, and will be offered in order of priority. The parameter with the smallest priority value will be the one listed first, and thus the one that is offered initially when creating a resource of this type in GATE Developer. For example, the following is a simplified extract from gate.corpora.DocumentImpl:

1@CreoleParameter(disjunction="src", priority=1) 
2public void setSourceUrl(URL src) { /* */ } 
3 
4@CreoleParameter(disjunction="src", priority=2) 
5public void setStringContent(String content) { /* */ }

This declares the parameters “stringContent” and “sourceUrl” as mutually-exclusive, and when creating an instance of this resource in GATE Developer the parameter that will be shown initially is sourceUrl. To set stringContent instead the user must select it from the drop-down list. Parameters with the same declared priority value will appear next to each other in the list, but their relative ordering is not specified. Parameters with no explicit priority are always listed after those that do specify a priority.

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

1  @Optional 
2  @RunTime 
3  @CreoleParameter 
4  public void setAnnotationSetName(String asName) { 
5    ...

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:

1  @HiddenCreoleParameter 
2  public void setSomeParam(String someParam) { 
3    super.setSomeParam(someParam); 
4  }

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:

1  // superclass 
2  @CreoleParameter(comment = "Location of the grammar file", 
3                   suffixes = "jape") 
4  public void setGrammarUrl(URL grammarLocation) { 
5    ... 
6  } 
7 
8  @Optional 
9  @RunTime 
10  @CreoleParameter(comment = "Feature to set on success") 
11  public void setSuccessFeature(String name) { 
12    ... 
13  }
1  // 
2  // subclass 
3 
4  // override the default value, inherit everything else 
5  @CreoleParameter(defaultValue = "resources/defaultGrammar.jape") 
6  public void setGrammarUrl(URL url) { 
7    super.setGrammarUrl(url); 
8  } 
9 
10  // we want the parameter to be required in the subclass 
11  @Optional(false) 
12  @CreoleParameter 
13  public void setSuccessFeature(String name) { 
14    super.setSuccessFeature(name); 
15  }

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:

<CREOLE-DIRECTORY>  
  <JAR SCAN="true">acmePlugin-1.0.jar</JAR>  
 
  <!-- Add the following to override the annotations -->  
  <RESOURCE>  
    <CLASS>com.acme.plugin.UsefulPR</CLASS>  
    <PARAMETER NAME="listUrl"  
      DEFAULT="resources/myList.txt">java.net.URL</PARAMETER>  
  </RESOURCE>  
</CREOLE-DIRECTORY>

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 necessity it may be better to define them in XML so other users can disable them without re-compiling the class:

<CREOLE-DIRECTORY>  
  <JAR SCAN="true">myPlugin.jar</JAR>  
 
  <RESOURCE>  
    <CLASS>com.acme.AutoPR</CLASS>  
    <AUTOINSTANCE>  
      <PARAM NAME="type" VALUE="Sentence" />  
    </AUTOINSTANCE>  
    <AUTOINSTANCE>  
      <PARAM NAME="type" VALUE="Paragraph" />  
    </AUTOINSTANCE>  
  </RESOURCE>  
</CREOLE-DIRECTORY>

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:

1package com.acme; 
2import gate.*; 
3import gate.creole.metadata.CreoleResource; 
4 
5@CreoleResource 
6public class MyPR implements LanguageAnalyser { 
7  ... 
8}
<!-- creole.xml -->  
<CREOLE-DIRECTORY>  
  <CREOLE>  
    <RESOURCE>  
      <NAME>My Processing Resource</NAME>  
      <CLASS>com.acme.MyPR</CLASS>  
      <COMMENT>...</COMMENT>  
      <PARAMETER NAME="annotationSetName"  
        RUNTIME="true" OPTIONAL="true">java.lang.String</PARAMETER>  
      <!--  
      don’t need to declare document and corpus parameters, they  
      are inherited from LanguageAnalyser  
      -->  
    </RESOURCE>  
  </CREOLE>  
</CREOLE-DIRECTORY>

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

4.7.4 Loading Third-Party Libraries using Apache Ivy [#]

With “simple” plugins most of the code is contained in a single jar or relies on just one or two thrid-party libraries which are easy to enumerate within creole.xml in order for them to be loaded into GATE when the plugin is loaded. More complex plugins can, however, rely on a large number of third-party libraries, each of which may have it’s own dependencies. In an attempt to simplify the management of third-party libraries, within CREOLE plugins, Apache Ivy can be used to specify the dependencies.

No attempt is made here to explain the workings of Ivy or the format of the ivy.xml file. For full details you should refer to the approprioate section of the Ivy manual.

Incorporating an Ivy file within a CREOLE plugin is as simple as referencing it from within creole.xml. Assumuing you have used the default filename of ivy.xml then you can reference it via a simple <IVY> element.

<CREOLE-DIRECTORY>  
  <JAR SCAN="true">myPlugin.jar</JAR>  
  <IVY/>  
</CREOLE-DIRECTORY>

If you have used an alternative filename then you can specify it as the text content of the <IVY> element. For example, if the filename is plugin-ivy.xml you would reference it as follows:

<CREOLE-DIRECTORY>  
  <JAR SCAN="true">myPlugin.jar</JAR>  
  <IVY>plugin-ivy.xml</IVY>  
</CREOLE-DIRECTORY>

When the plugin is loaded into GATE Ivy resolves the dependencies, downloads the appropriate libraries (if necessary) and then makes them available to the plugin. Once the plugin is loaded it behaves exactly the same as any other plugin.

Note that if you export an application (see Section 3.9.4) then to ensure that it is self-contained and useable within any processing environment the Ivy based dependencies are expanded; the libraries are downloaded into the plugin’s lib folder, appropriate entires are added to creole.xml and the <IVY> element is removed.

4.8 Tools: How to Add Utilities to GATE Developer [#]

Visual Resources allow a developer to provide a GUI to interact with a particular resource type (PR or LR), but sometimes it is useful to provide general utilities for use in the GATE Developer GUI that are not tied to any specific resource type. Examples include the annotation diff tool and the Groovy console (provided by the Groovy plugin), both of which are self-contained tools that display in their own top-level window. To support this, the CREOLE model has the concept of a tool.

A resource type is marked as a tool by using the <TOOL/> element in its creole.xml definition, or by setting tool = true if using the @CreoleResource annotation configuration style. If a resource is declared to be a tool, and written to implement the gate.gui.ActionsPublisher interface, then whenever an instance of the resource is created its published actions will be added to the “Tools” menu in GATE Developer.

Since the published actions of every instance of the resource will be added to the tools menu, it is best not to use this mechanism on resource types that can be instantiated by the user. The “tool” marker is best used in combination with the “private” flag (to hide the resource from the list of available types in the GUI) and one or more hidden autoinstance definitions to create a limited number of instances of the resource when its defining plugin is loaded. See the GroovySupport resource in the Groovy plugin for an example of this.

4.8.1 Putting Your Tools in a Sub-Menu [#]

If your plugin provides a number of tools (or a number of actions from the same tool) you may wish to organise your actions into one or more sub-menus, rather than placing them all on the single top-level tools menu. To do this, you need to put a special value into the actions returned by the tool’s getActions() method:

1action.putValue(GateConstants.MENU_PATH_KEY, 
2    new String[] {"Acme toolkit", "Statistics"});

The key must be GateConstants.MENU_PATH_KEY and the value must be an array of strings. Each string in the array represents the name of one level of sub-menus. Thus in the example above the action would be placed under “Tools Acme toolkit Statistics”. If no MENU_PATH_KEY value is provided the action will be placed directly on the Tools menu.

4.8.2 Adding Tools To Existing Resource Types [#]

While Visual Resources (VR) allow you to add new features to a particular resource they have a number of shortcomings. Firstly not every new feature will require a full VR; often a new entry on the resources right-click menu will suffice. More importantly new feautres added via a VR are only available while the VR is open. A Resource Helper is a form of Tool, as above, which can add new menu options to any existing resource type without requiring a VR.

A Resource Helper is defined in the same way as a Tool (by setting the tool = true feature of the @CreoleResource annotation and loaded via an autoinstance definition) but must also extend the gate.gui.ResourceHelper class. A Resource Helper can then return a set of actions for a given resource which will be added to its right-click menu. See the FastInfosetExporter resource in the Format_FastInfoset plugin for an example of how this works.

A Resource Helper may also make new API calls accessable to allow similar functionality to be made available to GATE Embedded, see Section 7.20 for more details on how this works.

1The JavaBeans spec allows is instead of get for properties of the primitive type boolean, but GATE does not support parameters with primitive types. Parameters of type java.lang.Boolean (the wrapper class) are permitted, but these have get accessors anyway.

2In this particular case, as the type is a collection, you would specify java.lang.Integer as the ITEM_CLASS_NAME.

3When registering a class using CreoleRegister.registerComponent the base URL against which defaults for URL parameters are resolved is not specified. In such a resource it may be better to use Class.getResource to construct the default URLs if no value is supplied for the parameter by the user.