| 1 | /* |
| 2 | * Copyright 2006-2007 the original author or authors. |
| 3 | * |
| 4 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 5 | * you may not use this file except in compliance with the License. |
| 6 | * You may obtain a copy of the License at |
| 7 | * |
| 8 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 9 | * |
| 10 | * Unless required by applicable law or agreed to in writing, software |
| 11 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 12 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 13 | * See the License for the specific language governing permissions and |
| 14 | * limitations under the License. |
| 15 | */ |
| 16 | package org.springframework.batch.item.database; |
| 17 | |
| 18 | import java.util.Iterator; |
| 19 | import java.util.List; |
| 20 | |
| 21 | import org.springframework.batch.item.ExecutionContext; |
| 22 | import org.springframework.batch.item.ItemReader; |
| 23 | import org.springframework.batch.item.ItemStream; |
| 24 | import org.springframework.batch.item.NoWorkFoundException; |
| 25 | import org.springframework.beans.factory.InitializingBean; |
| 26 | import org.springframework.util.Assert; |
| 27 | |
| 28 | /** |
| 29 | * <p> |
| 30 | * Convenience class for driving query item readers. Item readers of this type |
| 31 | * use a 'driving query' to return back a list of keys. A key can be defined as |
| 32 | * anything that can uniquely identify a record so that a more detailed record |
| 33 | * can be retrieved for each object. This allows a much smaller footprint to be |
| 34 | * stored in memory for processing. The following 'Customer' example table will |
| 35 | * help illustrate this: |
| 36 | * |
| 37 | * <pre> |
| 38 | * CREATE TABLE CUSTOMER ( |
| 39 | * ID BIGINT IDENTITY PRIMARY KEY, |
| 40 | * NAME VARCHAR(45), |
| 41 | * CREDIT FLOAT |
| 42 | * ); |
| 43 | * </pre> |
| 44 | * |
| 45 | * <p> |
| 46 | * A cursor based solution would simply open up a cursor over ID, NAME, and |
| 47 | * CREDIT, and move it from one to the next. This can cause issues on databases |
| 48 | * with pessimistic locking strategies. A 'driving query' approach would be to |
| 49 | * return only the ID of the customer, then use a separate DAO to retrieve the |
| 50 | * name and credit for each ID. This means that there will be a call to a |
| 51 | * separate DAO for each call to {@link ItemReader#read()}. |
| 52 | * </p> |
| 53 | * |
| 54 | * <p> |
| 55 | * Mutability: Because this base class cannot guarantee that the keys returned |
| 56 | * by subclasses are immutable, care should be taken to not modify a key value. |
| 57 | * Doing so would cause issues if a rollback occurs. For example, if a call to |
| 58 | * read() is made, and the returned key is modified, a rollback will cause the |
| 59 | * next call to read() to return the same object that was originally returned, |
| 60 | * since there is no way to create a defensive copy, and re-querying the |
| 61 | * database for all the keys would be too resource intensive. |
| 62 | * </p> |
| 63 | * |
| 64 | * |
| 65 | * @author Lucas Ward |
| 66 | */ |
| 67 | public class DrivingQueryItemReader implements ItemReader, InitializingBean, ItemStream { |
| 68 | |
| 69 | private boolean initialized = false; |
| 70 | |
| 71 | private List keys; |
| 72 | |
| 73 | private Iterator keysIterator; |
| 74 | |
| 75 | private int currentIndex = 0; |
| 76 | |
| 77 | private int lastCommitIndex = 0; |
| 78 | |
| 79 | private KeyCollector keyGenerator; |
| 80 | |
| 81 | private boolean saveState = false; |
| 82 | |
| 83 | public DrivingQueryItemReader() { |
| 84 | |
| 85 | } |
| 86 | |
| 87 | /** |
| 88 | * Initialize the input source with the provided keys list. |
| 89 | * |
| 90 | * @param keys |
| 91 | */ |
| 92 | public DrivingQueryItemReader(List keys) { |
| 93 | this.keys = keys; |
| 94 | this.keysIterator = keys.iterator(); |
| 95 | } |
| 96 | |
| 97 | /** |
| 98 | * Return the next key in the List. |
| 99 | * |
| 100 | * @return next key in the list if not index is not at the last element, |
| 101 | * null otherwise. |
| 102 | */ |
| 103 | public Object read() { |
| 104 | |
| 105 | if (keysIterator.hasNext()) { |
| 106 | currentIndex++; |
| 107 | return keysIterator.next(); |
| 108 | } |
| 109 | |
| 110 | return null; |
| 111 | } |
| 112 | |
| 113 | /** |
| 114 | * Get the current key. This method will return the same object returned by |
| 115 | * the last read() method. If no items have been read yet the ItemReader |
| 116 | * yet, then null will be returned. |
| 117 | * |
| 118 | * @return the current key. |
| 119 | */ |
| 120 | protected Object getCurrentKey() { |
| 121 | if (initialized && currentIndex > 0) { |
| 122 | return keys.get(currentIndex - 1); |
| 123 | } |
| 124 | |
| 125 | return null; |
| 126 | } |
| 127 | |
| 128 | /** |
| 129 | * Close the resource by setting the list of keys to null, allowing them to |
| 130 | * be garbage collected. |
| 131 | */ |
| 132 | public void close(ExecutionContext executionContext) { |
| 133 | initialized = false; |
| 134 | currentIndex = 0; |
| 135 | lastCommitIndex = 0; |
| 136 | keys = null; |
| 137 | keysIterator = null; |
| 138 | } |
| 139 | |
| 140 | /** |
| 141 | * Initialize the item reader by delegating to the subclass in order to |
| 142 | * retrieve the keys. |
| 143 | * |
| 144 | * @throws IllegalStateException if the keys list is null or initialized is |
| 145 | * true. |
| 146 | */ |
| 147 | public void open(ExecutionContext executionContext) { |
| 148 | |
| 149 | Assert.state(keys == null && !initialized, "Cannot open an already opened input source" |
| 150 | + ", call close() first."); |
| 151 | keys = keyGenerator.retrieveKeys(executionContext); |
| 152 | if (keys == null || keys.size() == 0) { |
| 153 | throw new NoWorkFoundException("KeyGenerator must return at least 1 key"); |
| 154 | } |
| 155 | keysIterator = keys.listIterator(); |
| 156 | initialized = true; |
| 157 | } |
| 158 | |
| 159 | public void update(ExecutionContext executionContext) { |
| 160 | if (saveState) { |
| 161 | Assert.notNull(executionContext, "ExecutionContext must not be null"); |
| 162 | if (getCurrentKey() != null) { |
| 163 | keyGenerator.updateContext(getCurrentKey(), executionContext); |
| 164 | } |
| 165 | } |
| 166 | } |
| 167 | |
| 168 | public void afterPropertiesSet() throws Exception { |
| 169 | Assert.notNull(keyGenerator, "The KeyGenerator must not be null."); |
| 170 | } |
| 171 | |
| 172 | /** |
| 173 | * Set the key generation strategy to use for this input source. |
| 174 | * |
| 175 | * @param keyGenerator |
| 176 | */ |
| 177 | public void setKeyCollector(KeyCollector keyGenerator) { |
| 178 | this.keyGenerator = keyGenerator; |
| 179 | } |
| 180 | |
| 181 | /** |
| 182 | * Mark is supported as long as this {@link ItemStream} is used in a |
| 183 | * single-threaded environment. The state backing the mark is a single |
| 184 | * counter, keeping track of the current position, so multiple threads |
| 185 | * cannot be accommodated. |
| 186 | * |
| 187 | * @see org.springframework.batch.item.ItemReader#mark() |
| 188 | */ |
| 189 | public void mark() { |
| 190 | lastCommitIndex = currentIndex; |
| 191 | } |
| 192 | |
| 193 | /* |
| 194 | * (non-Javadoc) |
| 195 | * |
| 196 | * @see org.springframework.batch.io.support.AbstractTransactionalIoSource#reset(org.springframework.batch.item.ExecutionContext) |
| 197 | */ |
| 198 | public void reset() { |
| 199 | keysIterator = keys.listIterator(lastCommitIndex); |
| 200 | currentIndex = lastCommitIndex; |
| 201 | } |
| 202 | |
| 203 | public void setSaveState(boolean saveState) { |
| 204 | this.saveState = saveState; |
| 205 | } |
| 206 | } |