Apache Solr Search Server (Core Javadocs).
Packages 
Package Description
org.apache.solr
Commonly reused classes and interfaces (deprecated package, do not add new classes)
org.apache.solr.analysis
Factories and classes specific to text analysis and the creation of TokenStreams
org.apache.solr.api
Commonly used classes for Solr V2 API.
org.apache.solr.client.solrj.embedded
SolrJ client implementations for embedded solr access.
org.apache.solr.cloud
Classes for dealing with ZooKeeper when operating in SolrCloud mode.
org.apache.solr.cloud.api.collections
Package related to internal implementations of the SolrCloud collections api
org.apache.solr.cloud.overseer
Classes for updating cluster state in SolrCloud mode.
org.apache.solr.cluster
This package contains the interfaces giving access to cluster state, including nodes, collections and the structure of the collections (shards and replicas).
org.apache.solr.cluster.events
Interfaces defining support for cluster-level event generation and processing.
org.apache.solr.cluster.events.impl
Default implementation of ClusterEventProducer.
org.apache.solr.cluster.placement
This package contains interfaces visible by plugins (i.e.
org.apache.solr.cluster.placement.impl
Implementation classes (not visible to plugins, subject to change at any time) for the interfaces in org.apache.solr.cluster.placement and to make them work with the rest of Solr.
org.apache.solr.cluster.placement.plugins
Sample plugin implementations.
org.apache.solr.core
Core classes implementing Solr internals and the management of SolrCores
org.apache.solr.core.backup
Core classes for Solr's Backup/Restore functionality
org.apache.solr.core.backup.repository
BackupRepository Providing backup/restore repository interfaces to plug different storage systems
org.apache.solr.core.snapshots
Core classes for Solr's persistent snapshots functionality
org.apache.solr.filestore
Implementation of Package Store.
org.apache.solr.handler
Concrete implementations of SolrRequestHandler
org.apache.solr.handler.admin
SolrRequestHandler implementations for powering he Solr Admin UI
org.apache.solr.handler.admin.api
V2 API implementations for "admin" APIs.
org.apache.solr.handler.api
V2 utilities useful for all API implementations.
org.apache.solr.handler.component
SearchComponent implementations for use in SearchHandler
org.apache.solr.handler.configsets
V2 API classes for performing CRUD operations on configsets.
org.apache.solr.handler.designer
The SchemaDesignerAPI and supporting classes.
org.apache.solr.handler.export
Solr's Export Handler Functionality
org.apache.solr.handler.loader
ContentStreamLoader implementations for use in ContentStreamHandlerBase implementations
org.apache.solr.handler.tagger
The TaggerRequestHandler and supporting classes.
org.apache.solr.highlight
SolrHighlighter API and related implementations and utilities
org.apache.solr.index
Indexing related classes.
org.apache.solr.internal.csv
Internal classes used for reading/writing CSV
org.apache.solr.legacy
Deprecated stuff!
org.apache.solr.logging
APIs related to capturing log event info in the LoggingHandler
org.apache.solr.logging.jul
JUL based implementation of LogWatcher
org.apache.solr.logging.log4j2
LOG4J2 based implementation of LogWatcher
org.apache.solr.metrics
The SolrCoreMetricManager is responsible for collecting metrics from SolrMetricProducer's and exposing metrics to SolrMetricReporter's.
org.apache.solr.metrics.reporters
This package houses 'pluggable' metric reporters that inherit from the SolrMetricReporter class.
org.apache.solr.metrics.reporters.jmx
This package contains components that support SolrJmxReporter.
org.apache.solr.metrics.reporters.solr
This package contains SolrMetricReporter implementations specific to SolrCloud reporting.
org.apache.solr.packagemanager
This package contains Package Manager (CLI) implementation
org.apache.solr.parser
Solr native variant of the Lucene Classic QueryParser
org.apache.solr.pkg
Package management implementation
org.apache.solr.query
Solr Queries
org.apache.solr.request
APIs and classes for dealing with Solr requests
org.apache.solr.request.json
JSON related classes
org.apache.solr.request.macro
Parameter substitution / macro expansion.
org.apache.solr.response
API and implementations of QueryResponseWriter for formatting Solr request responses
org.apache.solr.response.transform
APIs and implementations of DocTransformer for modifying documents in Solr request responses
org.apache.solr.rest
Solr RESTful APIs.
org.apache.solr.rest.schema
Provides RESTful API access to managed resources in the Solr Schema.
org.apache.solr.rest.schema.analysis
Analysis-related functionality for RESTful API access to managed resources related to the schema, such as stopwords, protected words, and synonyms.
org.apache.solr.schema
IndexSchema and FieldType implementations for powering schema.xml
org.apache.solr.search
APIs and classes for parsing and processing search requests
org.apache.solr.search.facet
APIs and classes for the JSON Facet API.
org.apache.solr.search.function
Solr implementations of ValueSource for function queries.
org.apache.solr.search.function.distance
Solr implementations of ValueSource for distance based function queries.
org.apache.solr.search.grouping
APIs and classes for implementing result grouping
org.apache.solr.search.grouping.collector
Grouping related Collectors
org.apache.solr.search.grouping.distributed
Internal classes used to implement distributed result grouping
org.apache.solr.search.grouping.distributed.command
Internal classes used to implement distributed result grouping
org.apache.solr.search.grouping.distributed.requestfactory
Internal classes used to implement distributed result grouping
org.apache.solr.search.grouping.distributed.responseprocessor
Internal classes used to implement distributed result grouping
org.apache.solr.search.grouping.distributed.shardresultserializer
Internal classes used to implement distributed result grouping
org.apache.solr.search.grouping.endresulttransformer
APIs and classes for transforming result grouping results into the appropriate response format
org.apache.solr.search.join
Classes related to joins.
org.apache.solr.search.mlt
APIs and classes for implementing MoreLikeThis (mlt) QueryParser.
org.apache.solr.search.neural
APIs and classes for implementing Neural (Dense Retrieval) QueryParsers.
org.apache.solr.search.similarities
Factories for various built-in Lucene ranking models.
org.apache.solr.search.stats
APIs and Classes implementing the Stats component used for document frequency calculations.
org.apache.solr.security
Commonly used classes for Solr security framework.
org.apache.solr.servlet
Servlet related classes for powering the Solr HTTP API
org.apache.solr.servlet.cache
Caching related classes used in the Solr HTTP API
org.apache.solr.spelling
APIs and classes used by the SpellCheckComponent
org.apache.solr.spelling.suggest
APIs and classes used by the SpellCheckComponent
org.apache.solr.spelling.suggest.fst
Factories for FST and Analyzing based Suggesters
org.apache.solr.spelling.suggest.jaspell
Factories for JaSpell based Suggesters
org.apache.solr.spelling.suggest.tst
Factories for TST based Suggesters
org.apache.solr.uninverting
Support for creating docvalues on-the-fly from the inverted index at runtime.
org.apache.solr.update
APIs and classes for managing index updates
org.apache.solr.update.processor
UpdateRequestProcessorFactory APIs and implementations for use in UpdateRequestProcessorChains
org.apache.solr.util
Common utility classes used throughout Solr
org.apache.solr.util.circuitbreaker
Support for triggering exceptions on excessive resource usage
org.apache.solr.util.configuration
Common Util APIs related to Solr configuration.
org.apache.solr.util.configuration.providers
TODO
org.apache.solr.util.hll
A fork of Java-HyperLogLog package tweaked not to depend on fastutil and with cleanups to make it lean and clean.
org.apache.solr.util.plugin
Common APIs related to implementing Solr plugins
org.apache.solr.util.stats
Utilities for collecting statistics
org.apache.solr.util.tracing
Tracing related utilities