小编ani*_*ban的帖子

Baur&King在他们的书中说:

隐式连接始终沿着多对一或一对一关联,而不是通过集合值关联.

[P 646,Ch 14]

但是当我在代码中执行此操作时,它会生成CROSS JOIN而不是INNER JOIN.

映射来自Member2(多对一) - > CLub.

但Club2没有关于会员的信息,并且Member2拥有Club2的外键.

我的疑问是

// Implicit: Find all UK club member who is female
Transaction t1 = HibernateUtil.begin();
Query query =
    HibernateUtil.getSession().createQuery("From Member2 m where m.club2.country = 'UK' ");
List<Member2> memList = query.list();
for (Member2 m : memList)
  System.out.println(m);
HibernateUtil.end(t1);

并且,Hibernate生成以下SQL查询:

Hibernate: 
    select
        member2x0_.member_id as member_i1_1_,
        member2x0_.club_id as club_id5_1_,
        member2x0_.member_age as member_a2_1_,
        member2x0_.member_name as member_n3_1_,
        member2x0_.member_sex as member_s4_1_ 
    from
        TBL_MEMBER2 member2x0_ cross 
    join …

我试图用反射打破BillPaugh Singleton解决方案,我能够做到但我可以在访问BillPaughSingleTon解决方案时看到两个构造函数.为什么这样 ？另外通过反复试验发现HelperClass中的行是造成这种情况的.为什么呢？

BillPaughClass

package creational.BillPaugh;

public class SingleTonBillPaugh
{
  private SingleTonBillPaugh instance;

  public static SingleTonBillPaugh getInstance()
  {
    return SingleTonHelper.instance;
  }

  private SingleTonBillPaugh()
  {
    System.out.println(Thread.currentThread().getName() + " instance is going to be created");
  }

  static class SingleTonHelper
  {
    private static final SingleTonBillPaugh instance = new SingleTonBillPaugh(); //if we remove this line, multiple constructor will not be there. But this line is needed for singleton. 
  }
}

使用Reflection破坏SingleTon.

package creational.BillPaugh;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public class BreakBillPaughUsingReflection
{
  public static void …

我试图找出(出于好奇)哪个线程实际上使单个构造函数被调用.好吧,我完全明白,每次执行Test程序时线程都会有所不同,只是想务实地检查一下.我试过了

dumpStack() 
printStackTrace()
getStackTrace()

but the does not give any hint of other class (!phew)

然后尝试通过放置wait()来停止最后一行的线程,以便使用jvisualvm检查线程转储,令人惊讶的是,我想要的信息也在那里丢失.

谁能告诉我如何为此实现解决方案？

Singleton Class,其getInstance()方法是从每个线程(10)的run()方法调用的.但是对于只有一个Thread调用,实际的构造函数将运行.

package creational;

public class SingleTonBillPaugh
{
  private SingleTonBillPaugh instance;

  public static SingleTonBillPaugh getInstance()
  {

    return SingleTonHelper.instance;
  }

  private SingleTonBillPaugh()
  {
    System.out.println("instance is going to be created");
  }

  private static class SingleTonHelper
  {

    private static final SingleTonBillPaugh instance = new SingleTonBillPaugh();
  }
}

单例和线程创建的测试类.注意:故意添加obj.wait()以使线程处于暂停模式,从而使它们能够在jvisualvm中进行查看.

package creational;

import java.util.concurrent.TimeUnit;

public class SingleTonBillPaughTest
{
  public static void main(String[] args)
  {
    Thread[] t = new …

我正在做我的开发工作,并想知道在代码实践,内存和时间效率方面哪一个可能是更好的选择？

public static void printtHashmap(Map<?,?> hm)   {
    for(Object key : hm.keySet())
    {
        System.out.println(key+" "+hm.get(key));
    }
}

public static void printHashMapByEntry(Map<?,?> hm) {
    for(Map.Entry<?,?> entry: hm.entrySet())
    {
        System.out.println(entry.getKey()+" "+entry.getValue());
    }