Java8 在我的 JPA EclipseLink 2.5.2 环境中不断做奇怪的事情。我不得不删除问题 https://stackoverflow.com/questions/26806183/java-8-sorting-behaviour 昨天，因为这种情况下的排序受到一种奇怪的 JPA 行为的影响 - 我通过在进行最终排序之前强制执行第一个排序步骤找到了解决该问题的方法。
仍然在带有 JPA Eclipselink 2.5.2 的 Java 8 中，以下代码有时无法在我的环境中排序(Linux、MacOSX，均使用 build 1.8.0_25-b17)。它在 JDK 1.7 环境中按预期工作。
public List<Document> getDocumentsByModificationDate() {
List<Document> docs=this.getDocuments();
LOGGER.log(Level.INFO,"sorting "+docs.size()+" by modification date");
Comparator<Document> comparator=new ByModificationComparator();
Collections.sort(docs,comparator);
return docs;
}
当从 JUnit 测试中调用时，上述函数可以正常工作。 在生产环境中调试时，我得到一个日志条目:
INFORMATION: sorting 34 by modification date
但是在 TimSort 中 nRemaining < 2 的 return 语句被命中 - 所以没有排序发生。 JPA 提供的 IndirectList(参见 What collections does jpa return?)被认为是空的。
static <T> void sort(T[] a, int lo, int hi, Comparator<? super T> c,
T[] work, int workBase, int workLen) {
assert c != null && a != null && lo >= 0 && lo <= hi && hi <= a.length;
int nRemaining = hi - lo;
if (nRemaining < 2)
return; // Arrays of size 0 and 1 are always sorted
此解决方法正确排序:
if (docs instanceof IndirectList) {
IndirectList iList = (IndirectList)docs;
Object sortTargetObject = iList.getDelegateObject();
if (sortTargetObject instanceof List<?>) {
List<Document> sortTarget=(List<Document>) sortTargetObject;
Collections.sort(sortTarget,comparator);
}
} else {
Collections.sort(docs,comparator);
}
问题:
这是 JPA Eclipselink 错误还是我通常可以在自己的代码中对此做些什么？
请注意 - 我还不能将软件更改为 Java8 源代码合规性。当前环境是 Java8 运行时。
我对这种行为感到惊讶 - 尤其令人恼火的是，测试用例在生产环境中运行正常，但在生产环境中出现问题。
在 https://github.com/WolfgangFahl/JPAJava8Sorting 有一个示例项目 它具有与原始问题类似的结构。
它包含一个 http://sscce.org/ JUnit 测试示例通过调用 em.clear() 从而分离所有对象并强制使用 IndirectList 使问题可重现。请参阅下面的这个 JUnit 案例以供引用。
急切获取:
// https://stackoverflow.com/questions/8301820/onetomany-relationship-is-not-working
@OneToMany(cascade = CascadeType.ALL, mappedBy = "parentFolder", fetch=FetchType.EAGER)
Unit 案例有效。如果在 JDK 8 中使用 FetchType.LAZY 或省略获取类型，则行为可能与 JDK 7 中不同(我现在必须检查一下)。 为什么会这样？ 此时我假设需要指定 Eager 获取或在要排序的列表上迭代一次，基本上是在排序之前手动获取。 还能做什么？
JUnit 测试
persistence.xml 和pom.xml 可以取自https://github.com/WolfgangFahl/JPAJava8Sorting 测试可以使用 MYSQL 数据库运行，也可以使用 DERBY(默认)在内存中运行
package com.bitplan.java8sorting;
import static org.junit.Assert.assertEquals;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.persistence.Access;
import javax.persistence.AccessType;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.FetchType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Persistence;
import javax.persistence.Query;
import javax.persistence.Table;
import org.eclipse.persistence.indirection.IndirectList;
import org.junit.Test;
/**
* Testcase for
* https://stackoverflow.com/questions/26816650/java8-collections-sort-sometimes-does-not-sort-jpa-returned-lists
* @author wf
*
*/
public class TestJPASorting {
// the number of documents we want to sort
public static final int NUM_DOCUMENTS = 3;
// Logger for debug outputs
protected static Logger LOGGER = Logger.getLogger("com.bitplan.java8sorting");
/**
* a classic comparator
* @author wf
*
*/
public static class ByNameComparator implements Comparator<Document> {
// @Override
public int compare(Document d1, Document d2) {
LOGGER.log(Level.INFO,"comparing " + d1.getName() + "<=>" + d2.getName());
return d1.getName().compareTo(d2.getName());
}
}
// Document Entity - the sort target
@Entity(name = "Document")
@Table(name = "document")
@Access(AccessType.FIELD)
public static class Document {
@Id
String name;
@ManyToOne
Folder parentFolder;
/**
* @return the name
*/
public String getName() {
return name;
}
/**
* @param name the name to set
*/
public void setName(String name) {
this.name = name;
}
/**
* @return the parentFolder
*/
public Folder getParentFolder() {
return parentFolder;
}
/**
* @param parentFolder the parentFolder to set
*/
public void setParentFolder(Folder parentFolder) {
this.parentFolder = parentFolder;
}
}
// Folder entity - owning entity for documents to be sorted
@Entity(name = "Folder")
@Table(name = "folder")
@Access(AccessType.FIELD)
public static class Folder {
@Id
String name;
// https://stackoverflow.com/questions/8301820/onetomany-relationship-is-not-working
@OneToMany(cascade = CascadeType.ALL, mappedBy = "parentFolder", fetch=FetchType.EAGER)
List<Document> documents;
/**
* @return the name
*/
public String getName() {
return name;
}
/**
* @param name the name to set
*/
public void setName(String name) {
this.name = name;
}
/**
* @return the documents
*/
public List<Document> getDocuments() {
return documents;
}
/**
* @param documents the documents to set
*/
public void setDocuments(List<Document> documents) {
this.documents = documents;
}
/**
* get the documents of this folder by name
*
* @return a sorted list of documents
*/
public List<Document> getDocumentsByName() {
List<Document> docs = this.getDocuments();
LOGGER.log(Level.INFO, "sorting " + docs.size() + " documents by name");
if (docs instanceof IndirectList) {
LOGGER.log(Level.INFO, "The document list is an IndirectList");
}
Comparator<Document> comparator = new ByNameComparator();
// here is the culprit - do or don't we sort correctly here?
Collections.sort(docs, comparator);
return docs;
}
/**
* get a folder example (for testing)
* @return - a test folder with NUM_DOCUMENTS documents
*/
public static Folder getFolderExample() {
Folder folder = new Folder();
folder.setName("testFolder");
folder.setDocuments(new ArrayList<Document>());
for (int i=NUM_DOCUMENTS;i>0;i--) {
Document document=new Document();
document.setName("test"+i);
document.setParentFolder(folder);
folder.getDocuments().add(document);
}
return folder;
}
}
/** possible Database configurations
using generic persistence.xml:
<?xml version="1.0" encoding="UTF-8"?>
<!-- generic persistence.xml which only specifies a persistence unit name -->
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
version="2.0">
<persistence-unit name="com.bitplan.java8sorting" transaction-type="RESOURCE_LOCAL">
<description>sorting test</description>
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
<!-- set programmatically -->
</properties>
</persistence-unit>
</persistence>
*/
// in MEMORY database
public static final JPASettings JPA_DERBY=new JPASettings("Derby","org.apache.derby.jdbc.EmbeddedDriver","jdbc:derby:memory:test-jpa;create=true","APP","APP");
// MYSQL Database
// needs preparation:
// create database testsqlstorage;
// grant all privileges on testsqlstorage to cm@localhost identified by 'secret';
public static final JPASettings JPA_MYSQL=new JPASettings("MYSQL","com.mysql.jdbc.Driver","jdbc:mysql://localhost:3306/testsqlstorage","cm","secret");
/**
* Wrapper class for JPASettings
* @author wf
*
*/
public static class JPASettings {
String driver;
String url;
String user;
String password;
String targetDatabase;
EntityManager entityManager;
/**
* @param driver
* @param url
* @param user
* @param password
* @param targetDatabase
*/
public JPASettings(String targetDatabase,String driver, String url, String user, String password) {
this.driver = driver;
this.url = url;
this.user = user;
this.password = password;
this.targetDatabase = targetDatabase;
}
/**
* get an entitymanager based on my settings
* @return the EntityManager
*/
public EntityManager getEntityManager() {
if (entityManager == null) {
Map<String, String> jpaProperties = new HashMap<String, String>();
jpaProperties.put("eclipselink.ddl-generation.output-mode", "both");
jpaProperties.put("eclipselink.ddl-generation", "drop-and-create-tables");
jpaProperties.put("eclipselink.target-database", targetDatabase);
jpaProperties.put("eclipselink.logging.level", "FINE");
jpaProperties.put("javax.persistence.jdbc.user", user);
jpaProperties.put("javax.persistence.jdbc.password", password);
jpaProperties.put("javax.persistence.jdbc.url",url);
jpaProperties.put("javax.persistence.jdbc.driver",driver);
EntityManagerFactory emf = Persistence.createEntityManagerFactory(
"com.bitplan.java8sorting", jpaProperties);
entityManager = emf.createEntityManager();
}
return entityManager;
}
}
/**
* persist the given Folder with the given entityManager
* @param em - the entityManager
* @param folderJpa - the folder to persist
*/
public void persist(EntityManager em, Folder folder) {
em.getTransaction().begin();
em.persist(folder);
em.getTransaction().commit();
}
/**
* check the sorting - assert that the list has the correct size NUM_DOCUMENTS and that documents
* are sorted by name assuming test# to be the name of the documents
* @param sortedDocuments - the documents which should be sorted by name
*/
public void checkSorting(List<Document> sortedDocuments) {
assertEquals(NUM_DOCUMENTS,sortedDocuments.size());
for (int i=1;i<=NUM_DOCUMENTS;i++) {
Document document=sortedDocuments.get(i-1);
assertEquals("test"+i,document.getName());
}
}
/**
* this test case shows that the list of documents retrieved will not be sorted if
* JDK8 and lazy fetching is used
*/
@Test
public void testSorting() {
// get a folder with a few documents
Folder folder=Folder.getFolderExample();
// get an entitymanager JPA_DERBY=inMemory JPA_MYSQL=Mysql disk database
EntityManager em=JPA_DERBY.getEntityManager();
// persist the folder
persist(em,folder);
// sort list directly created from memory
checkSorting(folder.getDocumentsByName());
// detach entities;
em.clear();
// get all folders from database
String sql="select f from Folder f";
Query query = em.createQuery(sql);
@SuppressWarnings("unchecked")
List<Folder> folders = query.getResultList();
// there should be exactly one
assertEquals(1,folders.size());
// get the first folder
Folder folderJPA=folders.get(0);
// sort the documents retrieved
checkSorting(folderJPA.getDocumentsByName());
}
}
最佳答案
好吧，这是一个完美的教学游戏，告诉您为什么程序员不应该扩展不是为子类化而设计的类。 “Effective Java”等书籍告诉您原因:当父类(super class)进化时，试图拦截每个方法以改变其行为的尝试将会失败。
在这里，IndirectList 扩展了 Vector 并覆盖了几乎所有方法来修改其行为，这是一个明显的反模式。现在，在 Java 8 中，基类已经进化。
从 Java 8 开始，接口(interface)可以有 default 方法，因此添加了 sort 之类的方法，与 Collections.sort 不同，它具有优势，实现可以覆盖该方法并提供更适合特定 interface 实现的实现。 Vector 这样做有两个原因:现在所有方法都是 同步 的契约也扩展到排序，并且优化的实现可以将其内部数组传递给 Arrays .sort 方法跳过以前实现中已知的复制操作(ArrayList 做同样的事情)。
为了即使对于现有代码也能立即获得此好处，Collections.sort 已经过改进。它委托(delegate)给 List.sort，后者将默认委托(delegate)给另一个方法，该方法实现了通过 toArray 和使用 TimSort 进行复制的旧行为。但是，如果 List 实现覆盖了 List.sort，它也会影响 Collections.sort 的行为。
interface method using internal
List.sort array w/o copying
Collections.sort ─────────────────> Vector.sort ─────────────────> Arrays.sort
关于Java 8 Collections.sort(有时)不对 JPA 返回列表进行排序，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26816650/