Seal `IndexReader` and `IndexReaderContext` (#12296)

lucene/CHANGES.txt

@@ -47,6 +47,10 @@ API Changes
 * GITHUB#12107: Remove deprecated KnnVectorField, KnnVectorQuery, VectorValues and
   LeafReader#getVectorValues. (Luca Cavanna)
 
+* GITHUB#12296: Make IndexReader and IndexReaderContext classes explicitly sealed.
+  They have already been runtime-checked to only be implemented by the specific classes
+  so this is effectively a non-breaking change.
+
 New Features
 ---------------------
 

lucene/core/src/java/org/apache/lucene/index/CompositeReader.java

@@ -44,7 +44,7 @@ import org.apache.lucene.store.Directory;
  * synchronization, you should <b>not</b> synchronize on the <code>IndexReader</code> instance; use
  * your own (non-Lucene) objects instead.
  */
-public abstract class CompositeReader extends IndexReader {
+public abstract non-sealed class CompositeReader extends IndexReader {
 
   private volatile CompositeReaderContext readerContext = null; // lazy init
 

lucene/core/src/java/org/apache/lucene/index/IndexReader.java

@@ -63,17 +63,13 @@ import org.apache.lucene.store.AlreadyClosedException;
  * synchronization, you should <b>not</b> synchronize on the <code>IndexReader</code> instance; use
  * your own (non-Lucene) objects instead.
  */
-public abstract class IndexReader implements Closeable {
+public abstract sealed class IndexReader implements Closeable permits CompositeReader, LeafReader {
 
   private boolean closed = false;
   private boolean closedByChild = false;
   private final AtomicInteger refCount = new AtomicInteger(1);
 
-  IndexReader() {
-    if (!(this instanceof CompositeReader || this instanceof LeafReader))
-      throw new Error(
-          "IndexReader should never be directly extended, subclass LeafReader or CompositeReader instead.");
-  }
+  IndexReader() {}
 
   /**
    * A utility class that gives hooks in order to help build a cache based on the data that is

lucene/core/src/java/org/apache/lucene/index/IndexReaderContext.java

@@ -22,17 +22,17 @@ import java.util.List;
  * A struct like class that represents a hierarchical relationship between {@link IndexReader}
  * instances.
  */
-public abstract class IndexReaderContext {
+public abstract sealed class IndexReaderContext permits CompositeReaderContext, LeafReaderContext {
   /** The reader context for this reader's immediate parent, or null if none */
   public final CompositeReaderContext parent;
   /**
-   * <code>true</code> if this context struct represents the top level reader within the
-   * hierarchical context
+   * {@code true} if this context struct represents the top level reader within the hierarchical
+   * context
    */
   public final boolean isTopLevel;
-  /** the doc base for this reader in the parent, <code>0</code> if parent is null */
+  /** the doc base for this reader in the parent, {@code 0} if parent is null */
   public final int docBaseInParent;
-  /** the ord for this reader in the parent, <code>0</code> if parent is null */
+  /** the ord for this reader in the parent, {@code 0} if parent is null */
   public final int ordInParent;
 
   // An object that uniquely identifies this context without referencing
@@ -41,8 +41,6 @@ public abstract class IndexReaderContext {
   final Object identity = new Object();
 
   IndexReaderContext(CompositeReaderContext parent, int ordInParent, int docBaseInParent) {
-    if (!(this instanceof CompositeReaderContext || this instanceof LeafReaderContext))
-      throw new Error("This class should never be extended by custom code!");
     this.parent = parent;
     this.docBaseInParent = docBaseInParent;
     this.ordInParent = ordInParent;

lucene/core/src/java/org/apache/lucene/index/LeafReader.java

@@ -41,7 +41,7 @@ import org.apache.lucene.util.Bits;
  * synchronization, you should <b>not</b> synchronize on the <code>IndexReader</code> instance; use
  * your own (non-Lucene) objects instead.
  */
-public abstract class LeafReader extends IndexReader {
+public abstract non-sealed class LeafReader extends IndexReader {
 
   private final LeafReaderContext readerContext = new LeafReaderContext(this);