Cache: Improve documentation and terminology

PiperOrigin-RevId: 312130813

library/common/src/main/java/com/google/android/exoplayer2/upstream/DataSpec.java

@@ -27,9 +27,7 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
-/**
+/** Defines a region of data in a resource. */
- * Defines a region of data.
- */
 public final class DataSpec {
 
   /**
@@ -298,22 +296,21 @@ public final class DataSpec {
     }
   }
 
-  /** The {@link Uri} from which data should be read. */
+  /** A {@link Uri} from which data belonging to the resource can be read. */
   public final Uri uri;
 
   /**
-   * The offset of the data located at {@link #uri} within an original resource.
+   * The offset of the data located at {@link #uri} within the resource.
    *
-   * <p>Equal to 0 unless {@link #uri} provides access to a subset of an original resource. As an
+   * <p>Equal to 0 unless {@link #uri} provides access to a subset of the resource. As an example,
-   * example, consider a resource that can be requested over the network and is 1000 bytes long. If
+   * consider a resource that can be requested over the network and is 1000 bytes long. If {@link
-   * {@link #uri} points to a local file that contains just bytes [200-300], then this field will be
+   * #uri} points to a local file that contains just bytes [200-300], then this field will be set to
-   * set to {@code 200}.
+   * {@code 200}.
    *
    * <p>This field can be ignored except for in specific circumstances where the absolute position
-   * in the original resource is required in a {@link DataSource} chain. One example is when a
+   * in the resource is required in a {@link DataSource} chain. One example is when a {@link
-   * {@link DataSource} needs to decrypt the content as it's read. In this case the absolute
+   * DataSource} needs to decrypt the content as it's read. In this case the absolute position in
-   * position in the original resource is typically needed to correctly initialize the decryption
+   * the resource is typically needed to correctly initialize the decryption algorithm.
-   * algorithm.
    */
   public final long uriPositionOffset;
 
@@ -353,11 +350,11 @@ public final class DataSpec {
   public final Map<String, String> httpRequestHeaders;
 
   /**
-   * The absolute position of the data in the full stream.
+   * The absolute position of the data in the resource.
    *
    * @deprecated Use {@link #position} except for specific use cases where the absolute position
-   *     within the original resource is required within a {@link DataSource} chain. Where the
+   *     within the resource is required within a {@link DataSource} chain. Where the absolute
-   *     absolute position is required, use {@code uriPositionOffset + position}.
+   *     position is required, use {@code uriPositionOffset + position}.
    */
   @Deprecated public final long absoluteStreamPosition;
 
@@ -370,8 +367,8 @@ public final class DataSpec {
   public final long length;
 
   /**
-   * A key that uniquely identifies the original stream. Used for cache indexing. May be null if the
+   * A key that uniquely identifies the resource. Used for cache indexing. May be null if the data
-   * data spec is not intended to be used in conjunction with a cache.
+   * spec is not intended to be used in conjunction with a cache.
    */
   @Nullable public final String key;
 

library/core/src/main/java/com/google/android/exoplayer2/offline/ProgressiveDownloader.java

@@ -134,7 +134,7 @@ public final class ProgressiveDownloader implements Downloader {
 
   @Override
   public void remove() {
-    dataSource.getCache().removeSpans(dataSource.getCacheKeyFactory().buildCacheKey(dataSpec));
+    dataSource.getCache().removeResource(dataSource.getCacheKeyFactory().buildCacheKey(dataSpec));
   }
 
   private static final class ProgressForwarder implements CacheUtil.ProgressListener {

library/core/src/main/java/com/google/android/exoplayer2/offline/SegmentDownloader.java

@@ -205,13 +205,13 @@ public abstract class SegmentDownloader<M extends FilterableManifest<M>> impleme
       M manifest = getManifest(dataSource, manifestDataSpec);
       List<Segment> segments = getSegments(dataSource, manifest, true);
       for (int i = 0; i < segments.size(); i++) {
-        cache.removeSpans(cacheKeyFactory.buildCacheKey(segments.get(i).dataSpec));
+        cache.removeResource(cacheKeyFactory.buildCacheKey(segments.get(i).dataSpec));
       }
     } catch (IOException e) {
       // Ignore exceptions when removing.
     } finally {
       // Always attempt to remove the manifest.
-      cache.removeSpans(cacheKeyFactory.buildCacheKey(manifestDataSpec));
+      cache.removeResource(cacheKeyFactory.buildCacheKey(manifestDataSpec));
     }
   }
 

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/Cache.java

@@ -24,7 +24,20 @@ import java.util.NavigableSet;
 import java.util.Set;
 
 /**
- * An interface for cache.
+ * A cache that supports partial caching of resources.
+ *
+ * <h3>Terminology</h3>
+ *
+ * <ul>
+ *   <li>A <em>resource</em> is a complete piece of logical data, for example a complete media file.
+ *   <li>A <em>cache key</em> uniquely identifies a resource. URIs are often suitable for use as
+ *       cache keys, however this is not always the case. URIs are not suitable when caching
+ *       resources obtained from a service that generates multiple URIs for the same underlying
+ *       resource, for example because the service uses expiring URIs as a form of access control.
+ *   <li>A <em>cache span</em> is a byte range within a resource, which may or may not be cached. A
+ *       cache span that's not cached is called a <em>hole span</em>. A cache span that is cached
+ *       corresponds to a single underlying file in the cache.
+ * </ul>
  */
 public interface Cache {
 
@@ -108,51 +121,45 @@ public interface Cache {
   void release();
 
   /**
-   * Registers a listener to listen for changes to a given key.
+   * Registers a listener to listen for changes to a given resource.
    *
    * <p>No guarantees are made about the thread or threads on which the listener is called, but it
    * is guaranteed that listener methods will be called in a serial fashion (i.e. one at a time) and
    * in the same order as events occurred.
    *
-   * @param key The key to listen to.
+   * @param key The cache key of the resource.
    * @param listener The listener to add.
-   * @return The current spans for the key.
+   * @return The current spans for the resource.
    */
   NavigableSet<CacheSpan> addListener(String key, Listener listener);
 
   /**
    * Unregisters a listener.
    *
-   * @param key The key to stop listening to.
+   * @param key The cache key of the resource.
    * @param listener The listener to remove.
    */
   void removeListener(String key, Listener listener);
 
   /**
-   * Returns the cached spans for a given cache key.
+   * Returns the cached spans for a given resource.
    *
-   * @param key The key for which spans should be returned.
+   * @param key The cache key of the resource.
    * @return The spans for the key.
    */
   NavigableSet<CacheSpan> getCachedSpans(String key);
 
-  /**
+  /** Returns the cache keys of all of the resources that are at least partially cached. */
-   * Returns all keys in the cache.
-   *
-   * @return All the keys in the cache.
-   */
   Set<String> getKeys();
 
   /**
    * Returns the total disk space in bytes used by the cache.
-   *
-   * @return The total disk space in bytes.
    */
   long getCacheSpace();
 
   /**
-   * A caller should invoke this method when they require data from a given position for a given
+   * A caller should invoke this method when they require data starting from a given position in a
-   * key.
+   * given resource.
    *
    * <p>If there is a cache entry that overlaps the position, then the returned {@link CacheSpan}
    * defines the file in which the data is stored. {@link CacheSpan#isCached} is true. The caller
@@ -168,8 +175,8 @@ public interface Cache {
    *
    * <p>This method may be slow and shouldn't normally be called on the main thread.
    *
-   * @param key The key of the data being requested.
+   * @param key The cache key of the resource.
-   * @param position The position of the data being requested.
+   * @param position The starting position in the resource from which data is required.
    * @return The {@link CacheSpan}.
    * @throws InterruptedException If the thread was interrupted.
    * @throws CacheException If an error is encountered.
@@ -183,8 +190,8 @@ public interface Cache {
    *
    * <p>This method may be slow and shouldn't normally be called on the main thread.
    *
-   * @param key The key of the data being requested.
+   * @param key The cache key of the resource.
-   * @param position The position of the data being requested.
+   * @param position The starting position in the resource from which data is required.
    * @return The {@link CacheSpan}. Or null if the cache entry is locked.
    * @throws CacheException If an error is encountered.
    */
@@ -198,8 +205,8 @@ public interface Cache {
    *
    * <p>This method may be slow and shouldn't normally be called on the main thread.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource being written.
-   * @param position The starting position of the data.
+   * @param position The starting position in the resource from which data will be written.
    * @param length The length of the data being written, or {@link C#LENGTH_UNSET} if unknown. Used
    *     only to ensure that there is enough space in the cache.
    * @return The file into which data should be written.
@@ -230,12 +237,12 @@ public interface Cache {
   void releaseHoleSpan(CacheSpan holeSpan);
 
   /**
-   * Removes all {@link CacheSpan CacheSpans} with the given key, deleting the underlying files.
+   * Removes all {@link CacheSpan CacheSpans} for a resource, deleting the underlying files.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource being removed.
    */
   @WorkerThread
-  void removeSpans(String key);
+  void removeResource(String key);
 
   /**
    * Removes a cached {@link CacheSpan} from the cache, deleting the underlying file.
@@ -248,34 +255,36 @@ public interface Cache {
   void removeSpan(CacheSpan span);
 
   /**
-   * Queries if a range is entirely available in the cache.
+   * Returns whether the specified range of data in a resource is fully cached.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource.
-   * @param position The starting position of the data.
+   * @param position The starting position of the data in the resource.
    * @param length The length of the data.
    * @return true if the data is available in the Cache otherwise false;
    */
   boolean isCached(String key, long position, long length);
 
   /**
-   * Returns the length of the cached data block starting from the {@code position} to the block end
+   * Returns the length of continuously cached data starting from {@code position}, up to a maximum
-   * up to {@code length} bytes. If the {@code position} isn't cached then -(the length of the gap
+   * of {@code maxLength}, of a resource. If {@code position} isn't cached then {@code -holeLength}
-   * to the next cached data up to {@code length} bytes) is returned.
+   * is returned, where {@code holeLength} is the length of continuously uncached data starting from
+   * {@code position}, up to a maximum of {@code maxLength}.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource.
-   * @param position The starting position of the data.
+   * @param position The starting position of the data in the resource.
-   * @param length The maximum length of the data to be returned.
+   * @param length The maximum length of the data or hole to be returned.
-   * @return The length of the cached or not cached data block length.
+   * @return The length of the continuously cached data, or {@code -holeLength} if {@code position}
+   *     isn't cached.
    */
   long getCachedLength(String key, long position, long length);
 
   /**
-   * Applies {@code mutations} to the {@link ContentMetadata} for the given key. A new {@link
+   * Applies {@code mutations} to the {@link ContentMetadata} for the given resource. A new {@link
-   * CachedContent} is added if there isn't one already with the given key.
+   * CachedContent} is added if there isn't one already for the resource.
    *
    * <p>This method may be slow and shouldn't normally be called on the main thread.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource.
    * @param mutations Contains mutations to be applied to the metadata.
    * @throws CacheException If an error is encountered.
    */
@@ -284,10 +293,10 @@ public interface Cache {
       throws CacheException;
 
   /**
-   * Returns a {@link ContentMetadata} for the given key.
+   * Returns a {@link ContentMetadata} for the given resource.
    *
-   * @param key The cache key for the data.
+   * @param key The cache key of the resource.
-   * @return A {@link ContentMetadata} for the given key.
+   * @return The {@link ContentMetadata} for the resource.
    */
   ContentMetadata getContentMetadata(String key);
 }

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheSpan.java

@@ -24,13 +24,9 @@ import java.io.File;
  */
 public class CacheSpan implements Comparable<CacheSpan> {
 
-  /**
+  /** The cache key that uniquely identifies the resource. */
-   * The cache key that uniquely identifies the original stream.
-   */
   public final String key;
-  /**
+  /** The position of the {@link CacheSpan} in the resource. */
-   * The position of the {@link CacheSpan} in the original stream.
-   */
   public final long position;
   /**
    * The length of the {@link CacheSpan}, or {@link C#LENGTH_UNSET} if this is an open-ended hole.
@@ -49,8 +45,8 @@ public class CacheSpan implements Comparable<CacheSpan> {
    * Creates a hole CacheSpan which isn't cached, has no last touch timestamp and no file
    * associated.
    *
-   * @param key The cache key that uniquely identifies the original stream.
+   * @param key The cache key that uniquely identifies the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @param length The length of the {@link CacheSpan}, or {@link C#LENGTH_UNSET} if this is an
    *     open-ended hole.
    */
@@ -61,8 +57,8 @@ public class CacheSpan implements Comparable<CacheSpan> {
   /**
    * Creates a CacheSpan.
    *
-   * @param key The cache key that uniquely identifies the original stream.
+   * @param key The cache key that uniquely identifies the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @param length The length of the {@link CacheSpan}, or {@link C#LENGTH_UNSET} if this is an
    *     open-ended hole.
    * @param lastTouchTimestamp The last touch timestamp, or {@link C#TIME_UNSET} if {@link

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CachedContent.java

@@ -21,14 +21,14 @@ import com.google.android.exoplayer2.util.Log;
 import java.io.File;
 import java.util.TreeSet;
 
-/** Defines the cached content for a single stream. */
+/** Defines the cached content for a single resource. */
 /* package */ final class CachedContent {
 
   private static final String TAG = "CachedContent";
 
-  /** The cache file id that uniquely identifies the original stream. */
+  /** The cache id that uniquely identifies the resource. */
   public final int id;
-  /** The cache key that uniquely identifies the original stream. */
+  /** The cache key that uniquely identifies the resource. */
   public final String key;
   /** The cached spans of this content. */
   private final TreeSet<SimpleCacheSpan> cachedSpans;
@@ -40,8 +40,8 @@ import java.util.TreeSet;
   /**
    * Creates a CachedContent.
    *
-   * @param id The cache file id.
+   * @param id The cache id of the resource.
-   * @param key The cache stream key.
+   * @param key The cache key of the resource.
    */
   public CachedContent(int id, String key) {
     this(id, key, DefaultContentMetadata.EMPTY);
@@ -106,13 +106,15 @@ import java.util.TreeSet;
   }
 
   /**
-   * Returns the length of the cached data block starting from the {@code position} to the block end
+   * Returns the length of continuously cached data starting from {@code position}, up to a maximum
-   * up to {@code length} bytes. If the {@code position} isn't cached then -(the length of the gap
+   * of {@code maxLength}. If {@code position} isn't cached, then {@code -holeLength} is returned,
-   * to the next cached data up to {@code length} bytes) is returned.
+   * where {@code holeLength} is the length of continuously un-cached data starting from {@code
+   * position}, up to a maximum of {@code maxLength}.
    *
    * @param position The starting position of the data.
-   * @param length The maximum length of the data to be returned.
+   * @param length The maximum length of the data or hole to be returned.
-   * @return the length of the cached or not cached data block length.
+   * @return The length of continuously cached data, or {@code -holeLength} if {@code position}
+   *     isn't cached.
    */
   public long getCachedBytesLength(long position, long length) {
     SimpleCacheSpan span = getSpan(position);

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CachedContentIndex.java

@@ -48,6 +48,7 @@ import java.security.NoSuchAlgorithmException;
 import java.security.SecureRandom;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
@@ -223,31 +224,35 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
   }
 
   /**
-   * Adds the given key to the index if it isn't there already.
+   * Adds a resource to the index, if it's not there already.
    *
-   * @param key The cache key that uniquely identifies the original stream.
+   * @param key The cache key of the resource.
-   * @return A new or existing CachedContent instance with the given key.
+   * @return The new or existing {@link CachedContent} corresponding to the resource.
    */
   public CachedContent getOrAdd(String key) {
     @Nullable CachedContent cachedContent = keyToContent.get(key);
     return cachedContent == null ? addNew(key) : cachedContent;
   }
 
-  /** Returns a CachedContent instance with the given key or null if there isn't one. */
+  /**
+   * Returns the {@link CachedContent} for a resource, or {@code null} if the resource is not
+   * present in the index.
+   *
+   * @param key The cache key of the resource.
+   */
   @Nullable
   public CachedContent get(String key) {
     return keyToContent.get(key);
   }
 
   /**
-   * Returns a Collection of all CachedContent instances in the index. The collection is backed by
+   * Returns a read only collection of all {@link CachedContent CachedContents} in the index.
-   * the {@code keyToContent} map, so changes to the map are reflected in the collection, and
+   *
-   * vice-versa. If the map is modified while an iteration over the collection is in progress
+   * <p>Subsequent changes to the index are reflected in the returned collection. If the index is
-   * (except through the iterator's own remove operation), the results of the iteration are
+   * modified whilst iterating over the collection, the result of the iteration is undefined.
-   * undefined.
    */
   public Collection<CachedContent> getAll() {
-    return keyToContent.values();
+    return Collections.unmodifiableCollection(keyToContent.values());
   }
 
   /** Returns an existing or new id assigned to the given key. */
@@ -261,7 +266,11 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     return idToKey.get(id);
   }
 
-  /** Removes {@link CachedContent} with the given key from index if it's empty and not locked. */
+  /**
+   * Removes a resource if its {@link CachedContent} is both empty and unlocked.
+   *
+   * @param key The cache key of the resource.
+   */
   public void maybeRemove(String key) {
     @Nullable CachedContent cachedContent = keyToContent.get(key);
     if (cachedContent != null && cachedContent.isEmpty() && !cachedContent.isLocked()) {
@@ -282,7 +291,7 @@ import org.checkerframework.checker.nullness.compatqual.NullableType;
     }
   }
 
-  /** Removes empty and not locked {@link CachedContent} instances from index. */
+  /** Removes all resources whose {@link CachedContent CachedContents} are empty and unlocked. */
   public void removeEmpty() {
     String[] keys = new String[keyToContent.size()];
     keyToContent.keySet().toArray(keys);

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -471,7 +471,7 @@ public final class SimpleCache implements Cache {
   }
 
   @Override
-  public synchronized void removeSpans(String key) {
+  public synchronized void removeResource(String key) {
     Assertions.checkState(!released);
     for (CacheSpan span : getCachedSpans(key)) {
       removeSpanInternal(span);

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCacheSpan.java

@@ -23,7 +23,7 @@ import java.io.File;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 
-/** This class stores span metadata in filename. */
+/** A {@link CacheSpan} that encodes metadata into the names of the underlying cache files. */
 /* package */ final class SimpleCacheSpan extends CacheSpan {
 
   /* package */ static final String COMMON_SUFFIX = ".exo";
@@ -42,7 +42,7 @@ import java.util.regex.Pattern;
    *
    * @param cacheDir The parent abstract pathname.
    * @param id The cache file id.
-   * @param position The position of the stored data in the original stream.
+   * @param position The position of the stored data in the resource.
    * @param timestamp The file timestamp.
    * @return The cache file.
    */
@@ -53,8 +53,8 @@ import java.util.regex.Pattern;
   /**
    * Creates a lookup span.
    *
-   * @param key The cache key.
+   * @param key The cache key of the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @return The span.
    */
   public static SimpleCacheSpan createLookup(String key, long position) {
@@ -64,8 +64,8 @@ import java.util.regex.Pattern;
   /**
    * Creates an open hole span.
    *
-   * @param key The cache key.
+   * @param key The cache key of the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @return The span.
    */
   public static SimpleCacheSpan createOpenHole(String key, long position) {
@@ -75,8 +75,8 @@ import java.util.regex.Pattern;
   /**
    * Creates a closed hole span.
    *
-   * @param key The cache key.
+   * @param key The cache key of the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @param length The length of the {@link CacheSpan}.
    * @return The span.
    */
@@ -190,8 +190,8 @@ import java.util.regex.Pattern;
   }
 
   /**
-   * @param key The cache key.
+   * @param key The cache key of the resource.
-   * @param position The position of the {@link CacheSpan} in the original stream.
+   * @param position The position of the {@link CacheSpan} in the resource.
    * @param length The length of the {@link CacheSpan}, or {@link C#LENGTH_UNSET} if this is an
    *     open-ended hole.
    * @param lastTouchTimestamp The last touch timestamp, or {@link C#TIME_UNSET} if {@link

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/CacheDataSourceTest.java

@@ -432,7 +432,7 @@ public final class CacheDataSourceTest {
     TestUtil.readExactly(cacheDataSource, 100);
 
     // Delete cached data.
-    cache.removeSpans(cacheDataSource.getCacheKeyFactory().buildCacheKey(unboundedDataSpec));
+    cache.removeResource(cacheDataSource.getCacheKeyFactory().buildCacheKey(unboundedDataSpec));
     assertCacheEmpty(cache);
 
     // Read the rest of the data.

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/SimpleCacheTest.java

@@ -236,7 +236,7 @@ public class SimpleCacheTest {
     addCache(simpleCache, KEY_2, 20, 10);
     simpleCache.releaseHoleSpan(holeSpan);
 
-    simpleCache.removeSpans(KEY_1);
+    simpleCache.removeResource(KEY_1);
     assertThat(simpleCache.getCachedSpans(KEY_1)).isEmpty();
     assertThat(simpleCache.getCachedSpans(KEY_2)).hasSize(1);
   }