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A Codec is the component that controls how every part of a Lucene segment is written to and read from disk. Swapping the codec changes the binary format of the index without touching any search or analysis logic. Lucene discovers codec implementations through Java’s ServiceLoader SPI mechanism. The codec name is written into every segment file so Lucene can load the right implementation when the segment is opened later.

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Built-in codecs

Lucene104Codec

The current default. Used automatically by every new IndexWriterConfig unless you override it. Combines Lucene104PostingsFormat, Lucene90DocValuesFormat, Lucene90StoredFieldsFormat, and more.

SimpleTextCodec

Writes every index structure as human-readable plain text. Extremely slow and large — intended only for debugging and understanding the index format.
SimpleTextCodec is marked @lucene.experimental and is for recreational use only. Never use it in production.

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Codec sub-formats

Codec is an abstract class that delegates each responsibility to a dedicated format object. You can extend FilterCodec to override only the sub-formats you care about.
MethodResponsibility
postingsFormat()Term dictionary and postings lists (doc IDs, positions, offsets, payloads)
docValuesFormat()Per-document numeric, binary, sorted, and sorted-set doc values
storedFieldsFormat()Stored field values retrieved at search time
termVectorsFormat()Term vectors (per-document term/position/offset data)
normsFormat()Per-field length normalization factors
liveDocsFormat()Bitset of non-deleted documents within a segment
compoundFormat()Optional bundling of segment files into a single .cfs compound file
pointsFormat()BKD-tree encoded numeric and geo points
knnVectorsFormat()HNSW-indexed dense float vectors for k-NN search

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Setting a codec on IndexWriterConfig

IndexWriterConfig.setCodec() accepts any Codec instance. The codec is applied to all new segments flushed or merged by that writer. 
import org.apache.lucene.codecs.Codec;
import org.apache.lucene.codecs.lucene104.Lucene104Codec;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.analysis.standard.StandardAnalyzer;

// Use the default codec explicitly (BEST_SPEED stored-field compression)
IndexWriterConfig config = new IndexWriterConfig(new StandardAnalyzer());
config.setCodec(new Lucene104Codec(Lucene104Codec.Mode.BEST_SPEED));
To switch to the highest compression for stored fields, use Mode.BEST_COMPRESSION: 
IndexWriterConfig config = new IndexWriterConfig(new StandardAnalyzer());
config.setCodec(new Lucene104Codec(Lucene104Codec.Mode.BEST_COMPRESSION));
Lucene104Codec.Mode.BEST_COMPRESSION uses a slower but more compact algorithm for stored fields. It is a good choice for large indexes where I/O is the bottleneck and CPU is available.

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PerFieldPostingsFormat

The default Lucene104Codec uses PerFieldPostingsFormat internally to route each field to a specific PostingsFormat. This lets you use, for example, a memory-mapped format for a high-traffic field while keeping others on the default disk-based format. Override getPostingsFormatForField in a Lucene104Codec subclass to apply per-field routing: 
import org.apache.lucene.codecs.PostingsFormat;
import org.apache.lucene.codecs.lucene104.Lucene104Codec;

public class MyCodec extends Lucene104Codec {

    private final PostingsFormat blockTreeFormat =
            PostingsFormat.forName("Lucene104");

    @Override
    public PostingsFormat getPostingsFormatForField(String field) {
        // Route a specific high-traffic field to a named format;
        // fall back to the default for everything else.
        if ("title".equals(field)) {
            return PostingsFormat.forName("DirectPostings");
        }
        return super.getPostingsFormatForField(field);
    }
}
The format name written into the index must match a registered PostingsFormat implementation at read time. If you use a custom format, register it via Java SPI (META-INF/services/org.apache.lucene.codecs.PostingsFormat) in your JAR.
Similarly, override getDocValuesFormatForField to apply per-field doc values formats, and getKnnVectorsFormatForField to tune HNSW vector parameters per field.

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Writing a custom PostingsFormat

1

Extend PostingsFormat

Subclass org.apache.lucene.codecs.PostingsFormat and supply a unique name that will be written into the index.
import org.apache.lucene.codecs.FieldsConsumer;
import org.apache.lucene.codecs.FieldsProducer;
import org.apache.lucene.codecs.PostingsFormat;
import org.apache.lucene.index.SegmentReadState;
import org.apache.lucene.index.SegmentWriteState;

public class MyPostingsFormat extends PostingsFormat {

    public MyPostingsFormat() {
        super("MyPostings"); // name written into the index
    }

    @Override
    public FieldsConsumer fieldsConsumer(SegmentWriteState state)
            throws java.io.IOException {
        // Return your write-side implementation
        throw new UnsupportedOperationException("implement me");
    }

    @Override
    public FieldsProducer fieldsProducer(SegmentReadState state)
            throws java.io.IOException {
        // Return your read-side implementation
        throw new UnsupportedOperationException("implement me");
    }
}
2

Register via SPI

Create META-INF/services/org.apache.lucene.codecs.PostingsFormat in your JAR and add your fully-qualified class name:
com.example.MyPostingsFormat
3

Wire it up

Plug your format into a codec subclass and set that codec on IndexWriterConfig.
public class MyCodec extends Lucene104Codec {
    @Override
    public PostingsFormat getPostingsFormatForField(String field) {
        return new MyPostingsFormat();
    }
}

IndexWriterConfig config = new IndexWriterConfig(new StandardAnalyzer());
config.setCodec(new MyCodec());

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Checking the default codec

You can inspect or change the process-wide default codec that new IndexWriterConfig instances receive: 
// Read the current default
Codec defaultCodec = Codec.getDefault(); // "Lucene104" in Lucene 11

// Override the process-wide default (affects all new IndexWriterConfigs)
Codec.setDefault(new Lucene104Codec(Lucene104Codec.Mode.BEST_COMPRESSION));

// Look up a codec by the name stored in the index
Codec byName = Codec.forName("SimpleText");

// List all available codec names on the classpath
java.util.Set<String> names = Codec.availableCodecs();

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