DynamoDb enhanced client: support UpdateExpressions in single-request update

Author: anasatirbasa

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/operations/UpdateItemOperation.java

@@ -270,20 +270,9 @@ public TransactWriteItem generateTransactWriteItem(TableSchema<T> tableSchema, O
     }
 
     /**
-     * Generates the final UpdateExpression by merging expressions from multiple sources:
-     * <ol>
-     *   <li><b>POJO attributes</b>: Creates SET/REMOVE actions for non-key attributes from the item POJO</li>
-     *   <li><b>Extension expressions</b>: Includes UpdateExpression from extensions (if any)</li>
-     *   <li><b>Request expressions</b>: Includes explicit UpdateExpression from the request (if any)</li>
-     * </ol>
-     * 
-     * <p><b>Conflict Detection:</b> All conflicts are detected server-side by DynamoDB and throw {@code DynamoDbException}.
-     * 
-     * <p><b>Attribute Filtering:</b>
-     * When {@code ignoreNulls} is false, null POJO attributes normally generate REMOVE actions.
-     * However, attributes referenced in extension expressions are automatically excluded from
-     * REMOVE actions to prevent DynamoDB conflicts between extension and POJO operations.
-     * 
+     * Merges UpdateExpressions from POJO attributes, extensions, and request with priority-based resolution.
+     * Attributes referenced in extension expressions are excluded from REMOVE actions to prevent conflicts.
+     *
      * @param tableMetadata metadata about the table structure
      * @param transformation write modification from extensions containing UpdateExpression
      * @param attributes non-key attributes from the POJO item

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/update/UpdateExpressionResolver.java

@@ -53,30 +53,14 @@ public static Builder builder() {
     }
 
     /**
-     * Merges UpdateExpressions from three sources in priority order:
-     * <ol>
-     *   <li><b>Item attributes</b>: SET/REMOVE actions from item POJO (lowest priority)</li>
-     *   <li><b>Extension expressions</b>: Override conflicting item actions (medium priority)</li>
-     *   <li><b>Request expressions</b>: Override all other sources (highest priority)</li>
-     * </ol>
-     *
-     * <p><b>Implementation steps:</b>
-     * <ol>
-     *   <li>Find attributes referenced in extension and request expressions to exclude them from removal</li>
-     *   <li>Generate SET actions for all non-null item attributes</li>
-     *   <li>Generate REMOVE actions for null item attributes, excluding those referenced in expressions</li>
-     *   <li>Combine item SET and REMOVE expressions into a single item expression</li>
-     *   <li>Merge extension expressions with item expression (extension overrides conflicting item actions)</li>
-     *   <li>Merge request expressions with combined result (request overrides all conflicting actions)</li>
-     * </ol>
-     *
-     * <p>Higher priority expressions win conflicts. REMOVE actions are filtered to prevent conflicts,
-     * even when {@code ignoreNulls} is false.
-     *
-     * <p><b>Backward compatibility:</b> This enhancement is not a breaking change. Without request
-     * expressions, behavior is identical to previous versions.
-     *
-     * <p><b>Exception scenarios:</b> DynamoDbException may be thrown when the same attribute is updated by multiple sources.
+     * Merges UpdateExpressions from three sources with priority: item attributes (lowest),
+     * extension expressions (medium), request expressions (highest).
+     * 
+     * <p><b>Steps:</b> Find attributes in expressions to exclude from removal → generate item SET/REMOVE actions
+     * → merge with extension expressions → merge with request expressions.
+     * 
+     * <p><b>Backward compatibility:</b> Without request expressions, behavior is identical to previous versions.
+     * <p><b>Exceptions:</b> DynamoDbException may be thrown when the same attribute is updated by multiple sources.
      *
      * @return merged UpdateExpression, or empty if no updates needed
      */

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/model/TransactUpdateItemEnhancedRequest.java

@@ -249,11 +249,9 @@ public Builder<T> item(T item) {
         /**
          * Specifies custom update operations using DynamoDB's native update expression syntax. Enables advanced modifications
          * like incrementing counters or modifying lists/sets.
-         * <p>
-         * <b>Precedence:</b> Request expressions (highest) > Extension expressions > Item attributes (lowest).
+         * <p><b>Precedence:</b> Request expressions (highest) > Extension expressions > Item attributes (lowest).
          * This method overrides any conflicting operations from item attributes or extensions.
-         * <p>
-         * <b>Backward compatible:</b> New feature that doesn't affect existing behavior when not used.
+         * <p><b>Backward compatible:</b> New feature that doesn't affect existing behavior when not used.
          *
          * @param updateExpression the update operations to perform
          * @return a builder of this type