Java tutorial
/** * (c) Copyright 2012 WibiData, Inc. * * See the NOTICE file distributed with this work for additional * information regarding copyright ownership. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package com.moz.fiji.schema; import java.io.Closeable; import java.io.Flushable; import java.io.IOException; import java.util.Collections; import java.util.Map; import com.google.common.base.Objects; import org.apache.avro.Schema; import org.apache.avro.util.WeakIdentityHashMap; import org.apache.commons.lang.builder.HashCodeBuilder; import com.moz.fiji.annotations.ApiAudience; import com.moz.fiji.annotations.ApiStability; import com.moz.fiji.annotations.Inheritance; import com.moz.fiji.schema.avro.SchemaTableBackup; import com.moz.fiji.schema.util.BytesKey; import com.moz.fiji.schema.util.Hasher; /** * The Fiji schema table, which contains the lookup table between schema IDs, hashes, and full * schemas. * * @see FijiMetaTable * @see FijiSystemTable */ @ApiAudience.Framework @ApiStability.Stable @Inheritance.Sealed public interface FijiSchemaTable extends Flushable, Closeable { /** * Looks up a schema ID given an Avro schema object. * * If the schema is unknown, allocates a new ID and stores the new schema mapping. * * @param schema The full schema to store in the table. * @return The schema ID. * @throws IOException on I/O error. */ long getOrCreateSchemaId(Schema schema) throws IOException; /** * Looks up a schema hash given an Avro schema object. * * If the schema is unknown, allocates a new ID and stores the new schema mapping. * * @param schema Avro schema to look up. * @return The schema hash. * @throws IOException on I/O error. */ BytesKey getOrCreateSchemaHash(Schema schema) throws IOException; /** * Computes a schema hash. * * @param schema The Avro schema to hash. * @return The hash of the schema */ BytesKey getSchemaHash(Schema schema); /** * Looks up a schema given an ID. * * @param schemaId Schema ID to look up. * @return Avro schema, or null if the schema ID is unknown. * @throws IOException on I/O error. */ Schema getSchema(long schemaId) throws IOException; /** * Looks up a schema given a hash. * * @param schemaHash Schema hash to look up. * @return Avro schema, or null if the schema hash is unknown. * @throws IOException on I/O error. */ Schema getSchema(BytesKey schemaHash) throws IOException; /** * Looks up a schema entry from a given ID. * * @param schemaId Schema ID to look up. * @return SchemaEntry matching the given ID or null if the ID is unknown. * @throws IOException on I/O error. */ SchemaEntry getSchemaEntry(long schemaId) throws IOException; /** * Looks up a schema entry from a given hash. * * @param schemaHash Schema hash to look up. * @return SchemaEntry matching the given hash or null if the hash is unknown. * @throws IOException on I/O error. */ SchemaEntry getSchemaEntry(BytesKey schemaHash) throws IOException; /** * Looks up a schema entry from a given Schema definition. * * @param schema Schema definition to look up. * @return SchemaEntry matching the given Schema or null if the schema is unknown. * @throws IOException on I/O error. */ SchemaEntry getSchemaEntry(Schema schema) throws IOException; /** Association between a schema and its ID. */ public static class SchemaEntry { private final long mId; private final BytesKey mHash; private final Schema mSchema; /** * Creates a new schema entry. * * @param id the schema ID * @param hash the schema hash * @param schema the Avro schema object */ @ApiAudience.Private public SchemaEntry(long id, BytesKey hash, Schema schema) { this.mId = id; this.mHash = hash; this.mSchema = schema; } /** @return the schema ID */ public long getId() { return this.mId; } /** @return the schema hash */ public BytesKey getHash() { return this.mHash; } /** @return the Avro schema object */ public Schema getSchema() { return this.mSchema; } /** {@inheritDoc} */ @Override public String toString() { return Objects.toStringHelper(SchemaEntry.class).add("id", this.mId).add("hash", this.mHash) .add("schema", this.mSchema).toString(); } /** {@inheritDoc} */ @Override public boolean equals(Object other) { if (!(other instanceof SchemaEntry)) { return false; } final SchemaEntry entry = (SchemaEntry) other; return (this.mId == entry.mId) && (this.mHash.equals(entry.mHash)) && (this.mSchema.equals(entry.mSchema)); } /** {@inheritDoc} */ @Override public int hashCode() { return new HashCodeBuilder().append(mId).append(mHash).append(mSchema).toHashCode(); } } /** * Cache providing an efficient mapping from Avro schema object to the schema hash. * * Computing the hash code of a schema is expensive as it serializes the Avro schema object * into JSON. */ @ApiAudience.Private static final class SchemaHashCache { /** * Underlying cache is a weak identity hash map: * <li> We must use object IDs since Schema.hashCode() and Schema.equals() implement a * comparison that ignores doc fields or default values. * <li> We must use a weak map to ensure the cache gets garbage collected properly. */ private final Map<Schema, BytesKey> mCache = Collections .synchronizedMap(new WeakIdentityHashMap<Schema, BytesKey>()); /** * Computes a hash of the specified Avro schema. * * Fiji currently uses MD5 sums (128 bits) of the schema JSON representation. * * @param schema Avro schema to compute a hash of. * @return Hash code as an array of bytes (16 bytes). */ public static byte[] hashSchema(Schema schema) { return Hasher.hash(schema.toString()); } /** * Hashes an Avro schema. * * @param schema Avro schema to hash. * @return the schema hash. */ public BytesKey getHash(Schema schema) { final BytesKey hash = mCache.get(schema); if (null != hash) { return hash; } final BytesKey newHash = new BytesKey(hashSchema(schema)); mCache.put(schema, newHash); return newHash; } } /** * Returns schema backup information in a form that can be directly written to a MetadataBackup * record. To read more about the avro type that has been specified to store this info, see * Layout.avdl * * @throws IOException on I/O error. * @return A list of schema table entries. */ SchemaTableBackup toBackup() throws IOException; /** * Restores the schema entries from the specified backup record. * * @param backup The schema entries from a MetadataBackup record. This consist of the schema * definition, schema id, and schema hash. * @throws IOException on I/O error. */ void fromBackup(SchemaTableBackup backup) throws IOException; }