/*- * See the file LICENSE for redistribution information. * * Copyright (c) 2002,2008 Oracle. All rights reserved. * * $Id: EntityCursor.java,v 1.1 2008/02/07 17:12:26 mark Exp $ */ package com.sleepycat.persist; import java.util.Iterator; import com.sleepycat.db.CursorConfig; import com.sleepycat.db.DatabaseException; import com.sleepycat.db.LockMode; import com.sleepycat.db.Transaction; import com.sleepycat.persist.model.Relationship; import com.sleepycat.persist.model.SecondaryKey; /** * Traverses entity values or key values and allows deleting or updating the * entity at the current cursor position. The value type (V) is either an * entity class or a key class, depending on how the cursor was opened. * *

{@code EntityCursor} objects are not thread-safe. Cursors * should be opened, used and closed by a single thread.

* *

Cursors are opened using the {@link EntityIndex#keys} and {@link * EntityIndex#entities} family of methods. These methods are available for * objects of any class that implements {@link EntityIndex}: {@link * PrimaryIndex}, {@link SecondaryIndex}, and the indices returned by {@link * SecondaryIndex#keysIndex} and {@link SecondaryIndex#subIndex}. A {@link * ForwardCursor}, which implements a subset of cursor operations, is also * available via the {@link EntityJoin#keys} and {@link EntityJoin#entities} * methods.

* *

Values are always returned by a cursor in key order, where the key is * defined by the underlying {@link EntityIndex}. For example, a cursor on a * {@link SecondaryIndex} returns values ordered by secondary key, while an * index on a {@link PrimaryIndex} or a {@link SecondaryIndex#subIndex} returns * values ordered by primary key.

* *

WARNING: Cursors must always be closed to prevent resource leaks * which could lead to the index becoming unusable or cause an * OutOfMemoryError. To ensure that a cursor is closed in the * face of exceptions, call {@link #close} in a finally block. For example, * the following code traverses all Employee entities and closes the cursor * whether or not an exception occurs:

* *

 * {@literal @Entity}
 * class Employee {
 *
 *     {@literal @PrimaryKey}
 *     long id;
 *
 *     {@literal @SecondaryKey(relate=MANY_TO_ONE)}
 *     String department;
 *
 *     String name;
 *
 *     private Employee() {}
 * }
 *
 * EntityStore store = ...
 *
 * {@code PrimaryIndex} primaryIndex =
 *     store.getPrimaryIndex(Long.class, Employee.class);
 *
 * {@code EntityCursor} cursor = primaryIndex.entities();
 * try {
 *     for (Employee entity = cursor.first();
 *                   entity != null;
 *                   entity = cursor.next()) {
 *         // Do something with the entity...
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

Initializing the Cursor Position

* *

When it is opened, a cursor is not initially positioned on any value; in * other words, it is uninitialized. Most methods in this interface initialize * the cursor position but certain methods, for example, {@link #current} and * {@link #delete}, throw {@link IllegalStateException} when called for an * uninitialized cursor.

* *

Note that the {@link #next} and {@link #prev} methods return the first or * last value respectively for an uninitialized cursor. This allows the loop * in the example above to be rewritten as follows:

* *

 * {@code EntityCursor} cursor = primaryIndex.entities();
 * try {
 *     Employee entity;
 *     while ((entity = cursor.next()) != null) {
 *         // Do something with the entity...
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

Cursors and Iterators

* *

The {@link #iterator} method can be used to return a standard Java {@code * Iterator} that returns the same values that the cursor returns. For * example:

* *

 * {@code EntityCursor} cursor = primaryIndex.entities();
 * try {
 *     {@code Iterator} i = cursor.iterator();
 *     while (i.hasNext()) {
 *          Employee entity = i.next();
 *         // Do something with the entity...
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

The {@link Iterable} interface is also extended by {@link EntityCursor} * to allow using the cursor as the target of a Java "foreach" statement:

* *

 * {@code EntityCursor} cursor = primaryIndex.entities();
 * try {
 *     for (Employee entity : cursor) {
 *         // Do something with the entity...
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

The iterator uses the cursor directly, so any changes to the cursor * position impact the iterator and vice versa. The iterator advances the * cursor by calling {@link #next()} when {@link Iterator#hasNext} or {@link * Iterator#next} is called. Because of this interaction, to keep things * simple it is best not to mix the use of an {@code EntityCursor} * {@code Iterator} with the use of the {@code EntityCursor} traversal methods * such as {@link #next()}, for a single {@code EntityCursor} object.

* *

Key Ranges

* *

A key range may be specified when opening the cursor, to restrict the * key range of the cursor to a subset of the complete range of keys in the * index. A {@code fromKey} and/or {@code toKey} parameter may be specified * when calling {@link EntityIndex#keys(Object,boolean,Object,boolean)} or * {@link EntityIndex#entities(Object,boolean,Object,boolean)}. The key * arguments may be specified as inclusive or exclusive values.

* *

Whenever a cursor with a key range is moved, the key range bounds will be * checked, and the cursor will never be positioned outside the range. The * {@link #first} cursor value is the first existing value in the range, and * the {@link #last} cursor value is the last existing value in the range. For * example, the following code traverses Employee entities with keys from 100 * (inclusive) to 200 (exclusive):

* *

 * {@code EntityCursor} cursor = primaryIndex.entities(100, true, 200, false);
 * try {
 *     for (Employee entity : cursor) {
 *         // Do something with the entity...
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

Duplicate Keys

* *

When using a cursor for a {@link SecondaryIndex}, the keys in the index * may be non-unique (duplicates) if {@link SecondaryKey#relate} is {@link * Relationship#MANY_TO_ONE MANY_TO_ONE} or {@link Relationship#MANY_TO_MANY * MANY_TO_MANY}. For example, a {@code MANY_TO_ONE} {@code * Employee.department} secondary key is non-unique because there are multiple * Employee entities with the same department key value. The {@link #nextDup}, * {@link #prevDup}, {@link #nextNoDup} and {@link #prevNoDup} methods may be * used to control how non-unique keys are returned by the cursor.

* *

{@link #nextDup} and {@link #prevDup} return the next or previous value * only if it has the same key as the current value, and null is returned when * a different key is encountered. For example, these methods can be used to * return all employees in a given department.

* *

{@link #nextNoDup} and {@link #prevNoDup} return the next or previous * value with a unique key, skipping over values that have the same key. For * example, these methods can be used to return the first employee in each * department.

* *

For example, the following code will find the first employee in each * department with {@link #nextNoDup} until it finds a department name that * matches a particular regular expression. For each matching department it * will find all employees in that department using {@link #nextDup}.

* *

 * {@code SecondaryIndex} secondaryIndex =
 *     store.getSecondaryIndex(primaryIndex, String.class, "department");
 *
 * String regex = ...;
 * {@code EntityCursor} cursor = secondaryIndex.entities();
 * try {
 *     for (Employee entity = cursor.first();
 *                   entity != null;
 *                   entity = cursor.nextNoDup()) {
 *         if (entity.department.matches(regex)) {
 *             while (entity != null) {
 *                 // Do something with the matching entities...
 *                 entity = cursor.nextDup();
 *             }
 *         }
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

Updating and Deleting Entities with a Cursor

* *

The {@link #update} and {@link #delete} methods operate on the entity at * the current cursor position. Cursors on any type of index may be used to * delete entities. For example, the following code deletes all employees in * departments which have names that match a particular regular expression:

* *

 * {@code SecondaryIndex} secondaryIndex =
 *     store.getSecondaryIndex(primaryIndex, String.class, "department");
 *
 * String regex = ...;
 * {@code EntityCursor} cursor = secondaryIndex.entities();
 * try {
 *     for (Employee entity = cursor.first();
 *                   entity != null;
 *                   entity = cursor.nextNoDup()) {
 *         if (entity.department.matches(regex)) {
 *             while (entity != null) {
 *                 cursor.delete();
 *                 entity = cursor.nextDup();
 *             }
 *         }
 *     }
 * } finally {
 *     cursor.close();
 * }

* *

Note that the cursor can be moved to the next (or previous) value after * deleting the entity at the current position. This is an important property * of cursors, since without it you would not be able to easily delete while * processing multiple values with a cursor. A cursor positioned on a deleted * entity is in a special state. In this state, {@link #current} will return * null, {@link #delete} will return false, and {@link #update} will return * false.

* *

The {@link #update} method is supported only if the value type is an * entity class (not a key class) and the underlying index is a {@link * PrimaryIndex}; in other words, for a cursor returned by one of the {@link * PrimaryIndex#entities} methods. For example, the following code changes all * employee names to uppercase:

* *

 * {@code EntityCursor} cursor = primaryIndex.entities();
 * try {
 *     for (Employee entity = cursor.first();
 *                   entity != null;
 *                   entity = cursor.next()) {
 *         entity.name = entity.name.toUpperCase();
 *         cursor.update(entity);
 *     }
 * } finally {
 *     cursor.close();
 * }

* * @author Mark Hayes */ public interface EntityCursor extends ForwardCursor { /** * Moves the cursor to the first value and returns it, or returns null if * the cursor range is empty. * *