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| <h1 class="page">Example 9 - Spell Checker Service using Declarative Services</h1> |
| <div class="paragraph"> |
| <p>The purpose of this example is to re-implement the spell checker service in Example 6, but to do so using Declarative Service. |
| Therefore you need to install the Declarative Services implementation into your OSGi framework.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The spell checker service of Example 6 was complex because it needed to aggregate all available dictionary services and monitor their dynamic availability. |
| In addition, the spell checker service’s own availability was dependent upon the availability of dictionary services; |
| in other words, the spell checker service had a dynamic, one-to-many dependency on dictionary services. |
| As it turns out, service dependencies are not managed at all by the OSGi framework and end up being the responsibility of the application developer. |
| Declarative Services tries to eliminate complex and error-prone service dependency handling by automating it. |
| To do this, Declarative Services parses XML files in a bundle that describes the components we want to create and their service dependencies. |
| Instead of writing a lot of complex code, we simply use annotations. |
| At build time the tooling creates a declarative XML file which is included in the bundle. |
| There is no need to write an Activator.</p> |
| </div> |
| <div class="paragraph"> |
| <p>We define the new spell checker service in a file called <code>SpellCheckerImpl.java</code> as follows:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre>/* |
| * Apache Felix OSGi tutorial. |
| **/ |
| |
| package tutorial.example9; |
| |
| import java.util.ArrayList; |
| import java.util.List; |
| import java.util.StringTokenizer; |
| |
| import org.apache.felix.examples.dictionaryservice.DictionaryService; |
| import org.apache.felix.examples.spellcheckservice.SpellCheckService; |
| import org.osgi.service.component.annotations.Component; |
| import org.osgi.service.component.annotations.Reference; |
| import org.osgi.service.component.annotations.ReferenceCardinality; |
| import org.osgi.service.component.annotations.ReferencePolicy; |
| |
| |
| /** |
| * This class re-implements the spell check service of Example 6. This service |
| * implementation behaves exactly like the one in Example 6, specifically, it |
| * aggregates all available dictionary services, monitors their dynamic |
| * availability, and only offers the spell check service if there are dictionary |
| * services available. The service implementation is greatly simplified, though, |
| * by using the Service Component Runtime. Notice that there is no OSGi references in the |
| * application code; instead, the annotations describe the service |
| * dependencies to the Service Component Runtime, which automatically manages them and it |
| * also automatically registers the spell check services as appropriate. |
| */ |
| @Component |
| public class SpellCheckServiceImpl implements SpellCheckService |
| { |
| /** |
| * List of service objects. |
| * |
| * This field is managed by the Service Component Runtime and updated |
| * with the current set of available dictionary services. |
| * At least one dictionary service is required. |
| */ |
| @Reference(policy=ReferencePolicy.DYNAMIC, cardinality=ReferenceCardinality.AT_LEAST_ONE) |
| private volatile List<DictionaryService> m_svcObjList; |
| |
| /** |
| * Checks a given passage for spelling errors. A passage is any number of |
| * words separated by a space and any of the following punctuation marks: |
| * comma (,), period (.), exclamation mark (!), question mark (?), |
| * semi-colon (;), and colon(:). |
| * |
| * @param passage |
| * the passage to spell check. |
| * @return An array of misspelled words or null if no words are misspelled. |
| */ |
| public String[] check( String passage ) |
| { |
| // No misspelled words for an empty string. |
| if ( ( passage == null ) || ( passage.length() == 0 ) ) |
| { |
| return null; |
| } |
| |
| List<String> errorList = new ArrayList<String>(); |
| |
| // Tokenize the passage using spaces and punctionation. |
| StringTokenizer st = new StringTokenizer( passage, " ,.!?;:" ); |
| |
| // Put the current set of services in a local field |
| // the field m_svcObjList might be modified concurrently |
| final List<DictionaryService> localServices = m_svcObjList; |
| |
| // Loop through each word in the passage. |
| while ( st.hasMoreTokens() ) |
| { |
| String word = st.nextToken(); |
| boolean correct = false; |
| |
| // Check each available dictionary for the current word. |
| for(final DictionaryService dictionary : localServices) { |
| if ( dictionary.checkWord( word ) ) |
| { |
| correct = true; |
| } |
| } |
| |
| // If the word is not correct, then add it |
| // to the incorrect word list. |
| if ( !correct ) |
| { |
| errorList.add( word ); |
| } |
| } |
| |
| // Return null if no words are incorrect. |
| if ( errorList.size() == 0 ) |
| { |
| return null; |
| } |
| |
| // Return the array of incorrect words. |
| return errorList.toArray( new String[errorList.size()] ); |
| } |
| }</pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Notice how much simpler this service implementation is when compared to the same service implemented in Example 6. |
| There are no references to OSGi interfaces in our application code and all tricky and complex code dealing with monitoring of services is handled for us.</p> |
| </div> |
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