Assumptions<\/h3>\n
Lets assume, that we need a filter that would allow us to reverse every word we have in a given field. So, if the input is “solr.pl” the output would be “lp.rlos”. It’s not the hardest example, but for the purpose of this entry it will be enough. One more thing – I decided to omit describing how to setup your IDE, how to compile your code, build jar and stuff like that. We will only focus on the code.<\/p>\n
Additional Information<\/h3>\n
Code, which is presented in this post was created using Solr 3.6<\/a> libraries, although you shouldn’t have much problems with compiling it with Solr 4 binaries. Keep in mind though that some slight modifications may be needed (in case something changes before Solr 4.0 release).<\/p>\n In order for Solr to be able to use our filter, we need two classes. The first class is the actual filter implementation, which will be responsible for handling the actual logic. The second class is the filter factory, which will be responsible for creating instances of the filter. Lets get it done then.<\/p>\n In order to implement our filter we will extends the TokenFilter<\/em> class from the org.apache.lucene.analysis<\/em> and we will override the incrementToken<\/em> method. This method returns a boolean<\/em> value – if a value is still available for processing in the token stream, this method should return true<\/em>, is the token in the token stream shouldn’t be further analyzed this method should return false<\/em>. The implementation should look like the one below:\n<\/p>\n A few words about some of the lines of code in the above implementation:<\/p>\n As I wrote earlier, in order for Solr to be able to use our filter, we need to implement filter factory class. Because, we don’t have any special configuration values and such, factory implementation should be very simple. We will extends BaseTokenFilterFactory<\/em> class from the org.apache.solr.analysis<\/em> package. The implementation can look like the following:\n<\/p>\n As you can see filter factory implementation is simple – we only needed to override a single create<\/em> method in which we instantiate our filter and return it.<\/p>\n After compilation and jar file preparation, we copy the jar to a directory Solr will be able to see it. We can do this by creating the lib<\/em> directory in the Solr home directory and then adding the following entry to the solrconfig.xml<\/em> file:\n<\/p>\n Then we change the schema.xml<\/em> file and we add a new field type that will use our filter:\n<\/p>\n It is worth to note, that as class<\/em> attribute value of the filter<\/em> tag we provide the full package and class names of the factory we created, not the filter itself. It is important to remember that, otherwise Solr will throw errors.<\/p>\n In order to show you that it works, I provide the following screen shot of the Solr administration panel:<\/p>\nWhat We Need<\/h3>\n
Filter<\/h3>\n
package pl.solr.analysis;\n\nimport java.io.IOException;\n\nimport org.apache.lucene.analysis.TokenFilter;\nimport org.apache.lucene.analysis.TokenStream;\nimport org.apache.lucene.analysis.tokenattributes.CharTermAttribute;\n\npublic final class ReverseFilter extends TokenFilter {\n private CharTermAttribute charTermAttr;\n\n protected ReverseFilter(TokenStream ts) {\n super(ts);\n this.charTermAttr = addAttribute(CharTermAttribute.class);\n }\n\n @Override\n public boolean incrementToken() throws IOException {\n if (!input.incrementToken()) {\n return false;\n }\n\n int length = charTermAttr.length();\n char[] buffer = charTermAttr.buffer();\n char[] newBuffer = new char[length];\n for (int i = 0; i < length; i++) {\n newBuffer[i] = buffer[length - 1 - i];\n }\n charTermAttr.setEmpty();\n charTermAttr.copyBuffer(newBuffer, 0, newBuffer.length);\n return true;\n }\n}<\/pre>\nDescription of the Above Implementation<\/h3>\n
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\n<\/em><\/li>\n
\n<\/em><\/li>\nFilter Factory<\/h3>\n
package pl.solr.analysis;\n\nimport org.apache.lucene.analysis.TokenStream;\nimport org.apache.solr.analysis.BaseTokenFilterFactory;\n\npublic class ReverseFilterFactory extends BaseTokenFilterFactory {\n @Override\n public TokenStream create(TokenStream ts) {\n return new ReverseFilter(ts);\n }\n}<\/pre>\nConfiguration<\/h3>\n
<lib dir=\"..\/lib\/\" regex=\"*.jar\" \/><\/pre>\n
<fieldType name=\"text_reversed\" class=\"solr.TextField\">\n <analyzer>\n <tokenizer class=\"solr.WhitespaceTokenizerFactory\"\/>\n <filter class=\"pl.solr.analysis.ReverseFilterFactory\" \/>\n <\/analyzer>\n<\/fieldType><\/pre>\n
Does it Work ?<\/h3>\n
![\"\"](\"http:\/\/solr.pl\/wp-content\/uploads\/2012\/05\/ReverseFilter1.png\" "\\"ReverseFilter\\"")
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