Java8 Collections.sort（有时）不会对 JPA 返回的列表进行排序

Java8 在我的 JPA EclipseLink 2.5.2 环境中不断执行奇怪的操作。我不得不删除这个问题https://stackoverflow.com/questions/26806183/java-8-sorting-behaviour昨天，因为这种情况下的排序受到奇怪的 JPA 行为的影响 - 我找到了一种解决方法，方法是在进行最终排序之前强制执行第一个排序步骤。

仍然在使用 JPA Eclipselink 2.5.2 的 Java 8 中，以下代码有时在我的环境中无法排序（Linux、MacOSX，均使用构建版本 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 测试调用时，上述函数可以正常工作。 在生产环境中调试时，我收到一条日志条目：

INFORMATION: sorting 34 by modification date

但在 TimSort 中，nRemaining jpa返回哪些集合？) 由 JPA 提供的被认为是空的。

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

此解决方法可以正确排序：

   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 错误吗？或者我通常可以在自己的代码中对此做些什么？

请注意 - 我还无法将软件更改为符合 Java8 源代码。当前环境是Java8运行时。

我对这种行为感到惊讶 - 尤其令人烦恼的是，测试用例运行正确，而在生产环境中却出现问题。

有一个示例项目位于https://github.com/WolfgangFahl/JPAJava8Sorting它具有与原始问题类似的结构。

它包含一个http://sscce.org/JUnit 测试的示例通过调用 em.clear() 来重现问题，从而分离所有对象并强制使用 IndirectList。请参阅下面的 JUnit 案例以供参考。

急切地获取：

// 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（我现在必须检查这一点）。为什么会这样？此时，我假设需要指定“急切获取”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 在内存中运行（默认）

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());
  }
}

嗯，这是一个完美的说教游戏，告诉你为什么程序员不应该扩展不是为子类化而设计的类。像《Effective Java》这样的书会告诉你原因：当超类演变时，拦截每个方法以改变其行为的尝试将会失败。

Here, IndirectList延伸Vector并重写几乎所有方法来修改其行为，这是一个明显的反模式。现在，随着 Java 8 的发展，基类已经发生了变化。

从 Java 8 开始，接口可以具有default方法等等方法sort添加的优点是，与Collections.sort，实现可以重写该方法并提供更适合特定的实现interface执行。Vector这样做有两个原因：现在所有方法都是合同synchronized也扩展到排序，优化的实现可以将其内部数组传递给Arrays.sort方法跳过先前实现中已知的复制操作（ArrayList做同样的事）。

即使对于现有代码，为了立即获得此好处，Collections.sort已改装。它委托给List.sort默认情况下，它将委托给另一个方法来实现旧的复制行为toArray并使用TimSort。但如果一个List实施覆盖List.sort它会影响到的行为Collections.sort以及。

                  interface method              using internal
                  List.sort                     array w/o copying
Collections.sort ─────────────────> Vector.sort ─────────────────> Arrays.sort