小编Iur*_*rii的帖子

我有几张包含大量数据的表(大约1亿条记录).所以我不能将这些数据存储在内存中,但是我希望使用类来传输这个结果集java.util.stream并将此流传递给另一个类.我读了关于Stream.of和Stream.Builder运算符,但它们是内存中的缓冲流.那么有什么方法可以解决这个问题吗？提前致谢.

更新#1

好吧,我用Google搜索并找到了jooq库.我不确定,但看起来它可能适用于我的测试用例.总而言之,我有几张包含大量数据的表格.我想流式传输我的结果集并将此流传输到另一个方法.像这样的东西:

// why return Stream<String>? Because my result set has String type
private Stream<Record> writeTableToStream(DataSource dataSource, String table) {

    Stream<Record> record = null;
    try (Connection connection = dataSource.getConnection()) {
        String sql = "select * from " + table;

        try (PreparedStatement pSt = connection.prepareStatement(sql)) {
            connection.setAutoCommit(false);
            pSt.setFetchSize(5000);
            ResultSet resultSet = pSt.executeQuery();
            //
            record = DSL.using(connection)
                    .fetch(resultSet).stream();
        }
    } catch (SQLException sqlEx) {
        logger.error(sqlEx);
    }

    return record;
}

可以请有人建议,我是否正确？谢谢.

更新#2

我在 …

我有下一堂课:

映射器

public interface DeviceTokensMapper {

    DeviceTokensMapper INSTANCE = Mappers.getMapper(DeviceTokensMapper.class);

    @Mappings({
            @Mapping(source = "tokenName", target = "tokenName"),
            @Mapping(source = "userOsType", target = "osType"),
    })

    DeviceTokensDTO toDeviceTokensDTO(DeviceTokens deviceTokens);
}

实体:

@Entity
public class DeviceTokens {

    @Id
    @Column(name="token_name", nullable = false)
    private String tokenName;

    @Column(name = "os", nullable = false)
    @Enumerated
    private UserOSType userOsType;

    public DeviceTokens() {}

    public DeviceTokens(String tokenName, UserOSType userOSType) {
        this.tokenName = tokenName;
        this.userOsType = userOSType;
    }

    public String getTokenName() {
        return tokenName;
    }

    public void setTokenName(String tokenName) { …

我有下一个xml文件:

<AutoTest>
 <Source>EBS FX</Source>
 <CreateCFF>No</CreateCFF>
 <FoXML descriptor="pb.fx.spotfwd.trade.feed" version="2.0">
   <FxSpotFwdTradeFeed>
     <FxSpotFwd feed_datetime="17-Dec-2014 10:20:09" 
       cpty_sds_id="EBS" match_id="L845586141217" original_trade_id_feed="L80107141217" 
       value_date="20141218" trade_id_external="001-002141880445/5862" match_sds_id="EBSFeedCpty" 
       counter_ccy="USD" trade_id_feed="107" trade_type="S" feed_source_id="80"    quoting_term="M" 
       deal_ccy="GBP" rate="1.5" trade_date="20141217" modified_by="automation"    cpty_side="B" counter_amt="1500000"
       smart_match="0" booking_status_id="10" trade_status_id="22" deal_amt="1000000"  trade_direction="B">
       <Notes />
     </FxSpotFwd>
 </FxSpotFwdTradeFeed>
 <TestCases />
 </FoXML>
</AutoTest>

如何使用sed 获取trade_id_external属性的值？
我尝试了这个表达方式:sed -n '/trade_id_external/s/.*=//p' ./file.xml 但没有运气

我尝试在Spring Data项目中配置OneToMany和ManyToOne映射但有一些问题.

所以我有两个实体:雇主和项目.一个雇主可以有很多项目.

实体类:

Employer.java

@Entity
@Table (name="Employer")
public class Employer {

    @Id
    @SequenceGenerator(name="my_seq", sequenceName="GLOBAL_SEQUENCE")
    @GeneratedValue(strategy = GenerationType.SEQUENCE ,generator="my_seq")
    @Column (name="employer_id")
    private int id;

    @Column (name="name")
    private String name;

    @JoinColumn (name="employer_id")
    @OneToMany(mappedBy = "employer", cascade = CascadeType.ALL, fetch = FetchType.EAGER)
    private Set<Project> project = new HashSet<Project>();

    ......................
}   

Project.java

@Entity
@Table (name="Project")
public class Project {

    @Id
    @SequenceGenerator(name="my_seq", sequenceName="GLOBAL_SEQUENCE")
    @GeneratedValue(strategy = GenerationType.SEQUENCE ,generator="my_seq")
    @Column (name="project_id")
    private int id;

    @Column (name="name")
    private String …

让我们想象我有下一堂课:

public class Service {
    public void transferMoney(Account fromAcct, Account toAcct, int amount) {
      synchronized (fromAcct) {
        synchronized (toAccount) { // could we use here only one synchronized block?
            fromAcct.credit(amount);
            toAccount.debit(amount);
        }
      }
    }
}

class Account {
  private int amount = 0;

  public void credit(int sum) {
    amount = amount + sum;
  }

  public void debit(int sum) {
    amount = amount - sum;
  }
}

例如,我知道我们只能在方法中改变状态fromAcct和toAcct对象transferMoney.那么我们可以用一个同步块重写我们的方法吗？

public class Service {
 private …

我有下一个spring rest控制器来处理我的异常：

@ExceptionHandler(ConstraintViolationException.class)
@ResponseStatus(HttpStatus.BAD_REQUEST)
public @ResponseBody
ExceptionResponseDTO hiberAnnotationExceptionHandler(ConstraintViolationException exception) {
    return  new ExceptionResponseDTO("Entity is null or we are out of range!", exception.getMessage(), ExceptionMapper.INSTANCE.exceptionToExceptionDTO(exception));
}

我的实体类：

@Entity
public class Device {

    @Id
    @Column(name="name", nullable = false)
    private String Name;

    @Column(name = "send_date")
    @NotNull
    private Date sendDate;
}

我尝试模拟ConstraintViolationException，因此在我的控制器中使用下一个代码：

Device d = new Device();
d.setName("sdsdsd");
d.setSendDate(null);
deviceRepository.save(d);

结果，我收到下一个异常：

[dispatcherServlet] ：？-具有路径[]的上下文中Servlet [dispatcherServlet]的Servlet.service（）引发异常[请求处理失败；嵌套的异常是org.springframework.transaction.TransactionSystemException：无法提交JPA事务。嵌套的异常是javax.persistence.RollbackException：根本原因提交事务时出错，原因是javax.validation.ConstraintViolationException：组[javax.validation.groups.Default，]的更新期间，对类[com.entity.Device]的验证失败违反约束的列表：[ConstraintViolationImpl {interpolatedMessage ='可能不为空'，propertyPath = sendDate，rootBeanClass = class com.entity.Device，messageTemplate ='{javax.validation.constraints.NotNull.message}'}]

因此，从堆栈跟踪中可以看到，我首先收到TransactionSystemException，因此，我的ExceptionHandler方法（hiberAnnotationExceptionHandler）不被调用。所以我的问题是如何模拟此异常（ …

我想使用 QueryDSL 库构建选择计数查询，如下所示：

select count(1) from table1

我创建了下一个代码：

SQLTemplates sqlTemplates = builder.printSchema().build();
Configuration configuration = new Configuration(sqlTemplates);
Path table = new RelationalPathBase(Object.class, "alias", "hr", "table1");
StringPath[] columnsPath = new StringPath[1];
columnsPath[0] = Expressions.stringPath(table, "field1");
SQLQuery sqlQuery = new SQLQuery(dataSource.getConnection(), configuration)
                .from(table);
sqlQuery.setUseLiterals(true);
String selectStatement = sqlQuery.getSQL(columnsPath).getSQL();

结果 selectStatement 是下一个：

select alias.field1 from hr.table1 alias

请有人建议如何重写上面的代码以获得

select count(alias.field1) from hr.table1 alias

我有一个方法,接受块的文件和大小,并返回分块文件的列表.但是我的文件行可能会被破坏的主要问题,例如在主文件中我有下一行:

|1|aaa|bbb|ccc|
|2|ggg|ddd|eee|

拆分后我可以在一个文件中:

|1|aaa|bbb

在另一个文件中:

|ccc|2|
|ggg|ddd|eee|

这是代码:

public static List<File> splitFile(File file, int sizeOfFileInMB) throws    IOException {
  int counter = 1;
  List<File> files = new ArrayList<>();

  int sizeOfChunk = 1024 * 1024 * sizeOfFileInMB;
  byte[] buffer = new byte[sizeOfChunk];

  try (BufferedInputStream bis = new BufferedInputStream(new FileInputStream(file))) {
    String name = file.getName();

    int tmp = 0;
    while ((tmp = bis.read(buffer)) > 0) {
        File newFile = new File(file.getParent(), name + "."
                + String.format("%03d", counter++));
        try (FileOutputStream out = new …

假设我有下一堂课：

public class Singleton{

  private static Singleton _instance; 

  public static Singleton getInstance(){

   if(_instance == null){
         synchronized(Singleton.class){
           if(_instance == null)
           _instance = new Singleton();
         }
   }
   return _instance;
 }

例如，我们有两个线程A和B，它们尝试同时执行getInstance()方法。我是否正确理解该过程：

1. 线程A进入getInstance()方法获取锁；
2. 线程B也进入getInstance()方法并被阻塞；
3. 线程A创建新的 Singleton() 对象并释放锁，现在_instance变量的最后一个值应该对线程B可见？或者线程B仍然可以拥有自己的_instance变量副本，该副本与主内存不同步（_instance =null）？