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Chapter 7
GATE Embedded [#]

7.1 Quick Start with GATE Embedded [#]

Embedding GATE-based language processing in other applications using GATE Embedded (the GATE API) is straightforward:

For example, this code will create the ANNIE extraction system:

1  // initialise the GATE library 
2  Gate.init(); 
4  // load ANNIE as an application from a gapp file 
5  SerialAnalyserController controller = (SerialAnalyserController) 
6    PersistenceManager.loadObjectFromFile(new File(new File( 
7      Gate.getPluginsHome(), ANNIEConstants.PLUGIN_DIR), 
8        ANNIEConstants.DEFAULT_FILE));

If you want to use resources from any plugins, you need to load the plugins before calling createResource:

1  Gate.init(); 
3  // need Tools plugin for the Morphological analyser 
4  Gate.getCreoleRegister().registerDirectories( 
5    new File(Gate.getPluginsHome(), "Tools").toURL() 
6  ); 
8  ... 
10  ProcessingResource morpher = (ProcessingResource) 
11    Factory.createResource("gate.creole.morph.Morph");

Instead of creating your processing resources individually using the Factory, you can create your application in GATE Developer, save it using the ‘save application state’ option (see Section 3.8.3), and then load the saved state from your code. This will automatically reload any plugins that were loaded when the state was saved, you do not need to load them manually.

1  Gate.init(); 
3  CorpusController controller = (CorpusController) 
4    PersistenceManager.loadObjectFromFile(new File("savedState.xgapp")); 
6  // loadObjectFromUrl is also available

There are many examples of using GATE Embedded available at http://gate.ac.uk/wiki/code-repository/.

7.2 Resource Management in GATE Embedded [#]

As outlined earlier, GATE defines three different types of resources:

Language Resources
: (LRs) entities that hold linguistic data.
Processing Resources
: (PRs) entities that process data.
Visual Resources
: (VRs) components used for building graphical interfaces.

These resources are collectively named CREOLE1 resources.

All CREOLE resources have some associated meta-data in the form of an entry in a special XML file named creole.xml. The most important role of that meta-data is to specify the set of parameters that a resource understands, which of them are required and which not, if they have default values and what those are. The valid parameters for a resource are described in the resource’s section of its creole.xml file or in Java annotations on the resource class – see Section 4.7.

All resource types have creation-time parameters that are used during the initialisation phase. Processing Resources also have run-time parameters that get used during execution (see Section 7.5 for more details).

Controllers are used to define GATE applications and have the role of controlling the execution flow (see Section 7.6 for more details).

This section describes how to create and delete CREOLE resources as objects in a running Java virtual machine. This process involves using GATE’s Factory class2, and, in the case of LRs, may also involve using a DataStore.

CREOLE resources are Java Beans; creation of a resource object involves using a default constructor, then setting parameters on the bean, then calling an init() method. The Factory takes care of all this, makes sure that the GATE Developer GUI is told about what is happening (when GUI components exist at runtime), and also takes care of restoring LRs from DataStores. A programmer using GATE Embedded should never call the constructor of a resource: always use the Factory!

Creating a resource involves providing the following information:

  Parameters and features need to be provided in the form of a GATE Feature Map which is essentially a java Map (java.util.Map) implementation, see Section 7.4.2 for more details on Feature Maps.

Creating a resource via the Factory involves passing values for any create-time parameters that require setting to the Factory’s createResource method. If no parameters are passed, the defaults are used. So, for example, the following code creates a default ANNIE part-of-speech tagger:

1Gate.getCreoleRegister().registerDirectories(new File( 
2  Gate.getPluginsHome(), ANNIEConstants.PLUGIN_DIR).toURI().toURL()); 
3FeatureMap params = Factory.newFeatureMap(); // empty map: default parameters 
4ProcessingResource tagger = (ProcessingResource) 
5  Factory.createResource("gate.creole.POSTagger", params);

Note that if the resource created here had any parameters that were both mandatory and had no default value, the createResource call would throw an exception. In this case, all the information needed to create a tagger is available in default values given in the tagger’s XML definition (in plugins/ANNIE/creole.xml):

  <COMMENT>Mark Hepple’s Brill-style POS tagger</COMMENT>  
  <PARAMETER NAME="document"  
    COMMENT="The document to be processed"  
  <PARAMETER NAME="rulesURL" DEFAULT="resources/heptag/ruleset"  
    COMMENT="The URL for the ruleset file"  

Here the two parameters shown are either ‘runtime’ parameters, which are set before a PR is executed, or have a default value (in this case the default rules file is distributed with GATE itself).

When creating a Document, however, the URL of the source for the document must be provided3. For example:

1URL u = new URL("http://gate.ac.uk/hamish/"); 
2FeatureMap params = Factory.newFeatureMap(); 
3params.put("sourceUrl", u); 
4Document doc = (Document) 
5  Factory.createResource("gate.corpora.DocumentImpl", params);

Note that the document created here is transient: when you quit the JVM the document will no longer exist. If you want the document to be persistent, you need to store it in a DataStore (see Section 7.4.5).

Apart from createResource() methods with different signatures, Factory also provides some shortcuts for common operations, listed in table 7.1.




Creates a new Feature Map (as used in the example above).

newDocument(String content)

Creates a new GATE Document starting from a String value that will be used to generate the document content.

newDocument(URL sourceUrl)

Creates a new GATE Document using the text pointed by an URL to generate the document content.

newDocument(URL sourceUrl, String encoding)

Same as above but allows the specification of an encoding to be used while downloading the document content.

newCorpus(String name)

creates a new GATE Corpus with a specified name.

Table 7.1: Factory Operations

GATE maintains various data structures that allow the retrieval of loaded resources. When a resource is no longer required, it needs to be removed from those structures in order to remove all references to it, thus making it a candidate for garbage collection. This is achieved using the  deleteResource(Resource res) method on Factory.

Simply removing all references to a resource from the user code will NOT be enough to make the resource collect-able. Not calling  Factory.deleteResource() will lead to memory leaks!

7.3 Using CREOLE Plugins [#]

As shown in the examples above, in order to use a CREOLE resource the relevant CREOLE plugin must be loaded. Processing Resources, Visual Resources and Language Resources other than Document, Corpus and DataStore all require that the appropriate plugin is first loaded. When using Document, Corpus or DataStore, you do not need to first load a plugin. The following API calls listed in table 7.2 are relevant to working with CREOLE plugins.

Class gate.Gate



public static void addKnownPlugin(URL pluginURL)

adds the plugin to the list of known plugins.

public static void removeKnownPlugin(URL pluginURL)

tells the system to ‘forget’ about one previously known directory. If the specified directory was loaded, it will be unloaded as well - i.e. all the metadata relating to resources defined by this directory will be removed from memory.

public static void addAutoloadPlugin(URL pluginUrl)

adds a new directory to the list of plugins that are loaded automatically at start-up.

public static void removeAutoloadPlugin(URL pluginURL)

tells the system to remove a plugin URL from the list of plugins that are loaded automatically at system start-up. This will be reflected in the user’s configuration data file.

Class gate.CreoleRegister

public void registerDirectories(URL directoryUrl)

loads a new CREOLE directory. The new plugin is added to the list of known plugins if not already there.

public void removeDirectory(URL directory)

unloads a loaded CREOLE plugin.

Table 7.2: Calls Relevant to CREOLE Plugins

7.4 Language Resources [#]

This section describes the implementation of documents and corpora in GATE.

7.4.1 GATE Documents

Documents are modelled as content plus annotations (see Section 7.4.4) plus features (see Section 7.4.2).

The content of a document can be any implementation of the  gate.DocumentContent interface; the features are <attribute, value> pairs stored a Feature Map. Attributes are String values while the values can be any Java object.

The annotations are grouped in sets (see section 7.4.3). A document has a default (anonymous) annotations set and any number of named annotations sets.

Documents are defined by the gate.Document interface and there is also a provided implementation:

: transient document. Can be stored persistently through Java serialisation.

Main Document functions are presented in table 7.3.

Content Manipulation



DocumentContent getContent()

Gets the Document content.

void edit(Long start, Long end, DocumentContent replacement)

Modifies the Document content.

void setContent(DocumentContent newContent)

Replaces the entire content.

Annotations Manipulation



public AnnotationSet getAnnotations()

Returns the default annotation set.

public AnnotationSet getAnnotations(String name)

Returns a named annotation set.

public Map getNamedAnnotationSets()

Returns all the named annotation sets.

void removeAnnotationSet(String name)

Removes a named annotation set.

Input Output

String toXml()

Serialises the Document in XML format.

String toXml(Set aSourceAnnotationSet, boolean includeFeatures)

Generates XML from a set of annotations only, trying to preserve the original format of the file used to create the document.

Table 7.3: gate.Document methods.

7.4.2 Feature Maps [#]

All CREOLE resources as well as the Controllers and the annotations can have attached meta-data in the form of Feature Maps.

A Feature Map is a Java Map (i.e. it implements the java.util.Map interface) and holds <attribute-name, attribute-value> pairs. The attribute names are Strings while the values can be any Java Objects.

The use of non-Serialisable objects as values is strongly discouraged.

Feature Maps are created using the gate.Factory.newFeatureMap() method.

The actual implementation for FeatureMaps is provided by the  gate.util.SimpleFeatureMapImpl class.

Objects that have features in GATE implement the gate.util.FeatureBearer interface which has only the two accessor methods for the object features: FeatureMap getFeatures() and void setFeatures(FeatureMap features).

G¯ etting a particular feature from an object

1Object obj; 
2String featureName = "length"; 
3if(obj instanceof FeatureBearer){ 
4  FeatureMap features = ((FeatureBearer)obj).getFeatures(); 
5  Object value = (features == null) ? null : 
6                                      features.get(featureName); 

7.4.3 Annotation Sets [#]

A GATE document can have one or more annotation layers — an anonymous one, (also called default), and as many named ones as necessary.

An annotation layer is organised as a Directed Acyclic Graph (DAG) on which the nodes are particular locations —anchors— in the document content and the arcs are made out of annotations reaching from the location indicated by the start node to the one pointed by the end node (see Figure 7.1 for an illustration). Because of the graph metaphor, the annotation layers are also called annotation graphs. In terms of Java objects, the annotation layers are represented using the Set paradigm as defined by the collections library and they are hence named annotation sets. The terms of annotation layer, graph and set are interchangeable and refer to the same concept when used in this book.


Figure 7.1: The Annotation Graph model.

An annotation set holds a number of annotations and maintains a series of indices in order to provide fast access to the contained annotations.

The GATE Annotation Sets are defined by the gate.AnnotationSet interface and there is a default implementation provided:

annotation set implementation used by transient documents.

The annotation sets are created by the document as required. The first time a particular annotation set is requested from a document it will be transparently created if it doesn’t exist.

Tables 7.4 and 7.5 list the most used Annotation Set functions.

Annotations Manipulation



Integer add(Long start, Long end, String type, FeatureMap features)

Creates a new annotation between two offsets, adds it to this set and returns its id.

Integer add(Node start, Node end, String type, FeatureMap features)

Creates a new annotation between two nodes, adds it to this set and returns its id.

boolean remove(Object o)

Removes an annotation from this set.




Node firstNode()

Gets the node with the smallest offset.

Node lastNode()

Gets the node with the largest offset.

Node nextNode(Node node)

Get the first node that is relevant for this annotation set and which has the offset larger than the one of the node provided.

Set implementation

Iterator iterator()

int size()

Table 7.4: gate.AnnotationSet methods (general purpose).


AnnotationSet get(Long offset)

Select annotations by offset. This returns the set of annotations whose start node is the least such that it is less than or equal to offset. If a positional index doesn’t exist it is created. If there are no nodes at or beyond the offset parameter then it will return null.

AnnotationSet get(Long startOffset, Long endOffset)

Select annotations by offset. This returns the set of annotations that overlap totally or partially with the interval defined by the two provided offsets. The result will include all the annotations that either:

  • start before the start offset and end strictly after it
  • start at a position between the start and the end offsets

AnnotationSet get(String type)

Returns all annotations of the specified type.

AnnotationSet get(Set types)

Returns all annotations of the specified types.

AnnotationSet get(String type, FeatureMap constraints)

Selects annotations by type and features.

Set getAllTypes()

Gets a set of java.lang.String objects representing all the annotation types present in this annotation set.

Table 7.5: gate.AnnotationSet methods (searching).

¯ terating from left to right over all annotations of a given type


1AnnotationSet annSet = ...; 
2String type = "Person"; 
3//Get all person annotations 
4AnnotationSet persSet = annSet.get(type); 
5//Sort the annotations 
6List persList = new ArrayList(persSet); 
7Collections.sort(persList, new gate.util.OffsetComparator()); 
9Iterator persIter = persList.iterator(); 

7.4.4 Annotations [#]

An annotation, is a form of meta-data attached to a particular section of document content. The connection between the annotation and the content it refers to is made by means of two pointers that represent the start and end locations of the covered content. An annotation must also have a type (or a name) which is used to create classes of similar annotations, usually linked together by their semantics.

An Annotation is defined by:

start node
a location in the document content defined by an offset.
end node
a location in the document content defined by an offset.
a String value.
(see Section 7.4.2).
an Integer value. All annotations IDs are unique inside an annotation set.

In GATE Embedded, annotations are defined by the gate.Annotation interface and implemented by the gate.annotation.AnnotationImpl class. Annotations exist only as members of annotation sets (see Section 7.4.3) and they should not be directly created by means of a constructor. Their creation should always be delegated to the containing annotation set.

7.4.5 GATE Corpora [#]

A corpus in GATE is a Java List (i.e. an implementation of java.util.List) of documents. GATE corpora are defined by the gate.Corpus interface and the following implementations are available:

used for transient corpora.
used for persistent corpora that are stored in a serial datastore (i.e. as a directory in a file system).

Apart from implementation for the standard List methods, a Corpus also implements the methods in table 7.6.



String getDocumentName(int index)

Gets the name of a document in this corpus.

List getDocumentNames()

Gets the names of all the documents in this corpus.

void populate(URL directory, FileFilter filter, String encoding, boolean recurseDirectories)

Fills this corpus with documents created on the fly from selected files in a directory. Uses a FileFilter to select which files will be used and which will be ignored. A simple file filter based on extensions is provided in the Gate distribution (gate.util.ExtensionFileFilter).

Table 7.6: gate.Corpus methods.

Creating a corpus from all XML files in a directory

1Corpus corpus = Factory.newCorpus("My XML Files"); 
2File directory = ...; 
3ExtensionFileFilter filter = new ExtensionFileFilter("XML files", "xml"); 
4URL url = directory.toURL(); 
5corpus.populate(url, filter, null, false);

Using a DataStore

Assuming that you have a DataStore already open called myDataStore, this code will ask the datastore to take over persistence of your document, and to synchronise the memory representation of the document with the disk storage:

Document persistentDoc = myDataStore.adopt(doc, mySecurity);  

When you want to restore a document (or other LR) from a datastore, you make the same createResource call to the Factory as for the creation of a transient resource, but this time you tell it the datastore the resource came from, and the ID of the resource in that datastore:

1  URL u = ....; // URL of a serial datastore directory 
2  SerialDataStore sds = new SerialDataStore(u.toString()); 
3  sds.open(); 
5  // getLrIds returns a list of LR Ids, so we get the first one 
6  Object lrId = sds.getLrIds("gate.corpora.DocumentImpl").get(0); 
8  // we need to tell the factory about the LRs ID in the data 
9  // store, and about which datastore it is in - we do this 
10  // via a feature map: 
11  FeatureMap features = Factory.newFeatureMap(); 
12  features.put(DataStore.LR_ID_FEATURE_NAME, lrId); 
13  features.put(DataStore.DATASTORE_FEATURE_NAME, sds); 
15  // read the document back 
16  Document doc = (Document) 
17    Factory.createResource("gate.corpora.DocumentImpl", features);

7.5 Processing Resources [#]

Processing Resources (PRs) represent entities that are primarily algorithmic, such as parsers, generators or ngram modellers.

They are created using the GATE Factory in manner similar the Language Resources. Besides the creation-time parameters they also have a set of run-time parameters that are set by the system just before executing them.

Analysers are a particular type of processing resources in the sense that they always have a document and a corpus among their run-time parameters.

The most used methods for Processing Resources are presented in table 7.7



void setParameterValue(String paramaterName, Object parameterValue)

Sets the value for a specified parameter. method inherited from gate.Resource

void setParameterValues(FeatureMap parameters)

Sets the values for more parameters in one step. method inherited from gate.Resource

Object getParameterValue(String paramaterName)

Gets the value of a named parameter of this resource. method inherited from gate.Resource

Resource init()

Initialise this resource, and return it. method inherited from gate.Resource

void reInit()

Reinitialises the processing resource. After calling this method the resource should be in the state it is after calling init. If the resource depends on external resources (such as rules files) then the resource will re-read those resources. If the data used to create the resource has changed since the resource has been created then the resource will change too after calling reInit().

void execute()

Starts the execution of this Processing Resource.

void interrupt()

Notifies this PR that it should stop its execution as soon as possible.

boolean isInterrupted()

Checks whether this PR has been interrupted since the last time its Executable.execute() method was called.

Table 7.7: gate.ProcessingResource methods.

7.6 Controllers [#]

Controllers are used to create GATE applications. A Controller handles a set of Processing Resources and can execute them following a particular strategy. GATE provides a series of serial controllers (i.e. controllers that run their PRs in sequence):

a serial controller that takes any kind of PRs.
a serial controller that only accepts Language Analysers as member PRs.
a serial controller that accepts all types of PRs and that allows the inclusion or exclusion of member PRs from the execution chain according to certain run-time conditions (currently features on the document being processed are used).
a serial controller that only accepts Language Analysers and that allows the conditional run of member PRs.

¯ reating an ANNIE application and running it over a corpus

1// load the ANNIE plugin 
2Gate.getCreoleRegister().registerDirectories(new File( 
3 Gate.getPluginsHome(), "ANNIE").toURI().toURL()); 
5// create a serial analyser controller to run ANNIE with 
6SerialAnalyserController annieController = 
7 (SerialAnalyserController) Factory.createResource( 
8     "gate.creole.SerialAnalyserController", 
9     Factory.newFeatureMap(), 
10     Factory.newFeatureMap(), "ANNIE"); 
12// load each PR as defined in ANNIEConstants 
13for(int i = 0; i < ANNIEConstants.PR_NAMES.length; i++) { 
14  // use default parameters 
15  FeatureMap params = Factory.newFeatureMap(); 
16  ProcessingResource pr = (ProcessingResource) 
17      Factory.createResource(ANNIEConstants.PR_NAMES[i], 
18                             params); 
19  // add the PR to the pipeline controller 
20  annieController.add(pr); 
21} // for each ANNIE PR 
23// Tell ANNIEs controller about the corpus you want to run on 
24Corpus corpus = ...; 
26// Run ANNIE 

7.7 Persistent Applications [#]

GATE Embedded allows the persistent storage of applications in a format based on XML serialisation. This is particularly useful for applications management and distribution. A developer can save the state of an application when he/she stops working on its design and continue developing it in a next session. When the application reaches maturity it can be deployed to the client site using the same method.

When an application (i.e. a Controller) is saved, GATE will actually only save the values for the parameters used to create the Processing Resources that are contained in the application. When the application is reloaded, all the PRs will be re-created using the saved parameters.

Many PRs use external resources (files) to define their behaviour and, in most cases, these files are identified using URLs. During the saving process, all the URLs are converted relative URLs based on the location of the application file. This way, if the resources are packaged together with the application file, the entire application can be reliably moved to a different location.

API access to application saving and loading is provided by means of two static methods on the gate.util.persistence.PersistenceManager class, listed in table 7.8.



public static void saveObjectToFile(Object obj, File file)

Saves the data needed to re-create the provided GATE object to the specified file. The Object provided can be any type of Language or Processing Resource or a Controller. The procedures may work for other types of objects as well (e.g. it supports most Collection types).

public static Object loadObjectFromFile(File file)

Parses the file specified (which needs to be a file created by the above method) and creates the necessary object(s) as specified by the data in the file. Returns the root of the object tree.

Table 7.8: Application Saving and Loading

¯ aving and loading a GATE application

1//Where to save the application? 
2File file = ...; 
3//What to save? 
4Controller theApplication = ...; 
8          saveObjectToFile(theApplication, file); 
9//delete the application 
11theApplication = null; 
14//load the application back 
15theApplication = gate.util.persistence.PersistenceManager. 
16                 loadObjectFromFile(file);

7.8 Ontologies

Starting from GATE version 3.1, support for ontologies has been added. Ontologies are nominally Language Resources but are quite different from documents and corpora and are detailed in chapter 14.

Classes related to ontologies are to be found in the gate.creole.ontology package and its sub-packages. The top level package defines an abstract API for working with ontologies while the sub-packages contain concrete implementations. A client program should only use the classes and methods defined in the API and never any of the classes or methods from the implementation packages.

The entry point to the ontology API is the gate.creole.ontology.Ontology interface which is the base interface for all concrete implementations. It provides methods for accessing the class hierarchy, listing the instances and the properties.

Ontology implementations are available through plugins. Before an ontology language resource can be created using the gate.Factory and before any of the classes and methods in the API can be used, one of the implementing ontology plugins must be loaded. For details see chapter 14.

7.9 Creating a New Annotation Schema [#]

An annotation schema (see Section 3.4.6) can be brought inside GATE through the creole.xml file. By using the AUTOINSTANCE element, one can create instances of resources defined in creole.xml. The gate.creole.AnnotationSchema (which is the Java representation of an annotation schema file) initializes with some predefined annotation definitions (annotation schemas) as specified by the GATE team.

Example from GATE’s internal creole.xml (in src/gate/resources/creole):

<!-- Annotation schema -->  
  <NAME>Annotation schema</NAME>  
  <COMMENT>An annotation type and its features</COMMENT>  
  <PARAMETER NAME="xmlFileUrl" COMMENT="The url to the definition file"  
    <PARAM NAME ="xmlFileUrl" VALUE="schema/AddressSchema.xml" />  
    <PARAM NAME ="xmlFileUrl" VALUE="schema/DateSchema.xml" />  
    <PARAM NAME ="xmlFileUrl" VALUE="schema/FacilitySchema.xml" />  
  <!-- etc. -->  

In order to create a gate.creole.AnnotationSchema object from a schema annotation file, one must use the gate.Factory class;

1FeatureMap params = new FeatureMap();\\ 
3AnnotationSchema annotSchema = \\ 
4Factory.createResurce("gate.creole.AnnotationSchema", params);

Note: All the elements and their values must be written in lower case, as XML is defined as case sensitive and the parser used for XML Schema inside GATE searches is case sensitive.

In order to be able to write XML Schema definitions, the ones defined in GATE (resources/creole/schema) can be used as a model, or the user can have a look at http://www.w3.org/2000/10/XMLSchema for a proper description of the semantics of the elements used.

Some examples of annotation schemas are given in Section 5.4.1.

7.10 Creating a New CREOLE Resource [#]

To create a new resource you need to:

GATE Developer helps you with this process by creating a set of directories and files that implement a basic resource, including a Java code file and a Makefile. This process is called ‘bootstrapping’.

For example, let’s create a new component called GoldFish, which will be a Processing Resource that looks for all instances of the word ‘fish’ in a document and adds an annotation of type ‘GoldFish’.

First start GATE Developer (see Section 2.2). From the ‘Tools’


Figure 7.2: BootStrap Wizard Dialogue

menu select ‘BootStrap Wizard’, which will pop up the dialogue in figure 7.2. The meaning of the data entry fields:

Now we need to compile the class and package it into a JAR file. The bootstrap wizard creates an Ant build file that makes this very easy – so long as you have Ant set up properly, you can simply run

ant jar

This will compile the Java source code and package the resulting classes into GoldFish.jar. If you don’t have your own copy of Ant, you can use the one bundled with GATE - suppose your GATE is installed at /opt/gate-5.0-snapshot, then you can use /opt/gate-5.0-snapshot/bin/ant jar to build.

You can now load this resource into GATE; see Section 3.6. The default Java code that was created for our GoldFish resource looks like this:

2 *  GoldFish.java 
3 * 
4 *  You should probably put a copyright notice here. Why not use the 
5 *  GNU licence? (See http://www.gnu.org/.) 
6 * 
7 *  hamish, 26/9/2001 
8 * 
9 *  $Id: howto.tex,v 1.130 2006/10/23 12:56:37 ian Exp $ 
10 */ 
12package sheffield.creole.example; 
14import java.util.*; 
15import gate.*; 
16import gate.creole.*; 
17import gate.util.*; 
20 * This class is the implementation of the resource GOLDFISH. 
21 */ 
22@CreoleResource(name = "GoldFish", 
23        comment = "Add a descriptive comment about this resource") 
24public class GoldFish extends AbstractProcessingResource 
25  implements ProcessingResource { 
28} // class GoldFish

The default XML configuration for GoldFish looks like this:

<!-- creole.xml GoldFish -->  
<!--  hamish, 26/9/2001 -->  
<!-- $Id: howto.tex,v 1.130 2006/10/23 12:56:37 ian Exp $ -->  
  <JAR SCAN="true">GoldFish.jar</JAR>  

The directory structure containing these files


Figure 7.3: BootStrap directory tree

is shown in figure 7.3. GoldFish.java lives in the src/sheffield/creole/example directory. creole.xml and build.xml are in the top GoldFish directory. The lib directory is for libraries; the classes directory is where Java class files are placed; the doc directory is for documentation. These last two, plus GoldFish.jar are created by Ant.

This process has the advantage that it creates a complete source tree and build structure for the component, and the disadvantage that it creates a complete source tree and build structure for the component. If you already have a source tree, you will need to chop out the bits you need from the new tree (in this case GoldFish.java and creole.xml) and copy it into your existing one.

See the example code at http://gate.ac.uk/wiki/code-repository/.

7.11 Adding Support for a New Document Format [#]

In order to add a new document format, one needs to extend the gate.DocumentFormat class and to implement an abstract method called:

1public void unpackMarkup(Document doc) throws 
2 DocumentFormatException

This method is supposed to implement the functionality of each format reader and to create annotations on the document. Finally the document’s old content will be replaced with a new one containing only the text between markups.

If one needs to add a new textual reader will extend the gate.corpora.TextualDocumentFormat and override the unpackMarkup(doc) method.

This class needs to be implemented under the Java bean specifications because it will be instantiated by GATE using Factory.createResource() method.

The init() method that one needs to add and implement is very important because in here the reader defines its means to be selected successfully by GATE. What one needs to do is to add some specific information into certain static maps defined in DocumentFormat class, that will be used at reader detection time.

After that, a definition of the reader will be placed into the one’s creole.xml file and the reader will be available to GATE.

We present for the rest of the section a complete three step example of adding such a reader. The reader we describe in here is an XML reader.

Step 1

Create a new class called XmlDocumentFormat that extends gate.corpora.TextualDocumentFormat.

Step 2

Implement the unpackMarkup(Document doc) which performs the required functionality for the reader. Add XML detection means in init() method:

1public Resource init() throws ResourceInstantiationException{ 
2  // Register XML mime type 
3  MimeType mime = new MimeType("text","xml"); 
4  // Register the class handler for this mime type 
5  mimeString2ClassHandlerMap.put(mime.getType()+ "/" + mime.getSubtype(), 
6                                                                        this); 
7  // Register the mime type with mine string 
8  mimeString2mimeTypeMap.put(mime.getType() + "/" + mime.getSubtype(), mime); 
9  // Register file suffixes for this mime type 
10  suffixes2mimeTypeMap.put("xml",mime); 
11  suffixes2mimeTypeMap.put("xhtm",mime); 
12  suffixes2mimeTypeMap.put("xhtml",mime); 
13  // Register magic numbers for this mime type 
14  magic2mimeTypeMap.put("<?xml",mime); 
15  // Set the mimeType for this language resource 
16  setMimeType(mime); 
17  return this; 
18}// init()

More details about the information from those maps can be found in Section 5.5.1

Step 3

Add the following creole definition in the creole.xml document.

      <NAME>My XML Document Format</NAME>  

More information on the operation of GATE’s document format analysers may be found in Section 5.5.

7.12 Using GATE Embedded in a Multithreaded Environment [#]

GATE Embedded can be used in multithreaded applications, so long as you observe a few restrictions. First, you must initialise GATE by calling Gate.init() exactly once in your application, typically in the application startup phase before any concurrent processing threads are started.

Secondly, you must not make calls that affect the global state of GATE (e.g. loading or unloading plugins) in more than one thread at a time. Again, you would typically load all the plugins your application requires at initialisation time. It is safe to create instances of resources in multiple threads concurrently.

Thirdly, it is important to note that individual GATE processing resources, language resources and controllers are by design not thread safe – it is not possible to use a single instance of a controller/PR/LR in multiple threads at the same time – but for a well written resource it should be possible to use several different instances of the same resource at once, each in a different thread. When writing your own resource classes you should bear the following in mind, to ensure that your resource will be useable in this way.

Of course, if you are writing a PR that is simply a wrapper around an external library that imposes these kinds of limitations there is only so much you can do. If your resource cannot be made safe you should document this fact clearly.

All the standard ANNIE PRs are safe when independent instances are used in different threads concurrently, as are the standard transient document, transient corpus and controller classes. A typical pattern of development for a multithreaded GATE-based application is:

7.13 Using GATE Embedded within a Spring Application [#]

GATE Embedded provides helper classes to allow GATE resources to be created and managed by the Spring framework. For Spring 2.0 or later, GATE Embedded provides a custom namespace handler that makes them extremely easy to use. To use this namespace, put the following declarations in your bean definition file:

<beans xmlns="http://www.springframework.org/schema/beans"  

You can have Spring initialise GATE:

  <gate:init gate-home="WEB-INF" user-config-file="WEB-INF/user.xml">  

To create a GATE resource, use the <gate:resource> element.

  <gate:resource id="sharedOntology" scope="singleton"  
      <entry key="rdfXmlURL">  
        <value type="org.springframework.core.io.Resource"  

If you are familiar with Spring you will see that <gate:parameters> uses the same format as the standard <map> element, but values whose type is a Spring Resource will be converted to URLs before being passed to the GATE resource.

You can load a GATE saved application with

  <gate:saved-application location="WEB-INF/application.gapp" scope="prototype">  
      <gate:set-parameter pr-name="custom transducer" name="ontology"  
                          ref="sharedOntology" />  

‘Customisers’ are used to customise the application after it is loaded. In the example above, we load a singleton copy of an ontology which is then shared between all the separate instances of the (prototype) application. The <gate:set-parameter> customiser accepts all the same ways to provide a value as the standard Spring <property> element (a ”value” or ”ref” attribute, or a sub-element - <value>, <list>, <bean>, <gate:resource> …).

The <gate:add-pr> customiser provides support for the case where most of the application is in a saved state, but we want to create one or two extra PRs with Spring (maybe to inject other Spring beans as init parameters) and add them to the pipeline.

  <gate:saved-application ...>  
      <gate:add-pr add-before="OrthoMatcher" ref="myPr" />  

By default, the <gate:add-pr> customiser adds the target PR at the end of the pipeline, but an add-before or add-after attribute can be used to specify the name of a PR before (or after) which this PR should be placed. Alternatively, an index attribute places the PR at a specific (0-based) index into the pipeline. The PR to add can be specified either as a ‘ref’ attribute, or with a nested <bean> or <gate:resource> element.

These custom elements all define various factory beans. For full details, see the JavaDocs for gate.util.spring (the factory beans) and gate.util.spring.xml (the gate: namespace handler).

Note: the former approach using factory methods of the gate.util.spring.SpringFactory class will still work, but should be considered deprecated in favour of the new factory beans.

7.14 Using GATE Embedded within a Tomcat Web Application [#]

Embedding GATE in a Tomcat web application involves several steps.

  1. Put the necessary JAR files (gate.jar and all or most of the jars in gate/lib) in your webapp/WEB-INF/lib.
  2. Put the plugins that your application depends on in a suitable location (e.g. webapp/WEB-INF/plugins).
  3. Create suitable gate.xml configuration files for your environment.
  4. Set the appropriate paths in your application before calling Gate.init().

This process is detailed in the following sections.

7.14.1 Recommended Directory Structure

You will need to create a number of other files in your web application to allow GATE to work:

In this guide, we assume the following layout:


7.14.2 Configuration Files

Your gate.xml (the ‘site-wide configuration file’) should be as simple as possible:

<?xml version="1.0" encoding="UTF-8" ?>  
  <GATECONFIG Save_options_on_exit="false"  
              Save_session_on_exit="false" />  

Similarly, keep the user-gate.xml (the ‘user config file’) simple:

<?xml version="1.0" encoding="UTF-8" ?>  
  <GATECONFIG Known_plugin_path=";"  
              Load_plugin_path=";" />  

This way, you can control exactly which plugins are loaded in your webapp code.

7.14.3 Initialization Code

Given the directory structure shown above, you can initialize GATE in your web application like this:

1// imports 
3public class MyServlet extends HttpServlet { 
4  private static boolean gateInited = false; 
6  public void init() throws ServletException { 
7    if(!gateInited) { 
8      try { 
9        ServletContext ctx = getServletContext(); 
11        // use /path/to/your/webapp/WEB-INF as gate.home 
12        File gateHome = new File(ctx.getRealPath("/WEB-INF")); 
14        Gate.setGateHome(gateHome); 
15        // thus webapp/WEB-INF/plugins is the plugins directory, and 
16        // webapp/WEB-INF/gate.xml is the site config file. 
18        // Use webapp/WEB-INF/user-gate.xml as the user config file, to avoid 
19        // confusion with your own user config. 
20        Gate.setUserConfigFile(new File(gateHome, "user-gate.xml")); 
22        Gate.init(); 
23        // load plugins, for example... 
24        Gate.getCreoleRegister().registerDirectories( 
25          ctx.getResource("/WEB-INF/plugins/ANNIE")); 
27        gateInited = true; 
28      } 
29      catch(Exception ex) { 
30        throw new ServletException("Exception initialising GATE", 
31                                   ex); 
32      } 
33    } 
34  } 

Once initialized, you can create GATE resources using the Factory in the usual way (for example, see Section 7.1 for an example of how to create an ANNIE application). You should also read Section 7.12 for important notes on using GATE Embedded in a multithreaded application.

Instead of an initialization servlet you could also consider doing your initialization in a ServletContextListener, or using Spring (see Section 7.13).

7.15 Groovy Scripting for GATE [#]

Groovy is a dynamic programming language based on Java. You can use it as a scripting language for GATE, via the Groovy Console. Groovy is documented at http://groovy.codehaus.org/.

Groovy support is intended for users with programing skills, and with some knowledge of the GATE API. Groovy support is not enabled in GATE by default. In order to enable it, you must download and install Groovy in GATE, as follows.

  1. Download the Groovy distribution from http://groovy.codehaus.org/Download. GATE has been tested with Groovy 1.6.4
  2. Unzip the Groovy distribution - it doesn’t matter where, you do not need all of it.
  3. From the unzipped distribution, copy groovy-1.x.y/embeddable/groovy-all-1.x.y.jar in to the gate/lib directory in your GATE installation (where 1.x.y is the version number of Groovy).
  4. Groovy support will be enabled next time you start GATE.

Groovy support is currently available via a Groovy Console. To use it, open the console using the ‘Groovy Console’ item in the GATE tools menu. You can use then use the Groovy Console to write, load, and execute the Groovy language, as described in the documentation at http://groovy.codehaus.org/.

To help scripting GATE in Groovy, the following variable bindings are available from the Groovy Console.

Here’s an example script. It finds all documents with a feature ‘annotator’ set to ‘fred’, and puts them in a new corpus called ‘fredsDocs’.

3  docs.findAll{ 
4    it.getFeatures().get("annotator").equals("fred") 
5  } 

Why won’t the ‘Groovy executing’ dialog go away? Sometimes, when you execute a Groovy script through the console, a dialog will appear, saying ‘Groovy is executing. Please wait’. The dialog fails to go away even when the script has ended, and cannot be closed by clicking the ‘Interrupt’ button. You can, however, continue to use the Groovy Console, and the dialog will usually go away next time you run a script. This is not a GATE problem: it is a Groovy problem.

7.16 Saving Config Data to gate.xml

Arbitrary feature/value data items can be saved to the user’s gate.xml file via the following API calls:

To get the config data: Map configData = Gate.getUserConfig().

To add config data simply put pairs into the map: configData.put("my new config key", "value");.

To write the config data back to the XML file: Gate.writeUserConfig();.

Note that new config data will simply override old values, where the keys are the same. In this way defaults can be set up by putting their values in the main gate.xml file, or the site gate.xml file; they can then be overridden by the user’s gate.xml file.

7.17 Annotation merging through the API [#]

If we have annotations about the same subject on the same document from different annotators, we may need to merge those annotations to form a unified annotation. Two approaches for merging annotations are implemented in the API, via static methods in the class gate.util.AnnotationMerging.

The two methods have very similar input and output parameters. Each of the methods takes an array of annotation sets, which should be the same annotation type on the same document from different annotators, as input. A single feature can also be specified as a parameter (or given asnull if no feature is to be specified).

The output is a map, the key of which is one merged annotation and the value of which represents the annotators (in terms of the indices of the array of annotation sets) who support the annotation. The methods also have a boolean input parameter to indicate whether or not the annotations from different annotators are based on the same set of instances, which can be determined by the static method public boolean isSameInstancesForAnnotators(AnnotationSet[] annsA) in the class gate.util.IaaCalculation. One instance corresponds to all the annotations with the same span. If the annotation sets are based on the same set of instances, the merging methods will ensure that the merged annotations are on the same set of instances.

The two methods corresponding to those described for the Annotation Merging plugin described in Section 19.11. They are:

1CREOLE stands for Collection of REusable Objects for Language Engineering

2Fully qualified name: gate.Factory

3Alternatively a string giving the document source may be provided.

4See Section 4.4.