org.h2.store

类 LobStorageBackend

java.lang.Object

org.h2.store.LobStorageBackend

所有已实现的接口:

LobStorageInterface

public class LobStorageBackend
extends Object
implements LobStorageInterface

This class stores LOB objects in the database, in tables. This is the back-end i.e. the server side of the LOB storage.

Using the system session

Why do we use the system session to store the data? Some LOB operations can take a very long time. If we did them on a normal session, we would be locking the LOB tables for long periods of time, which is extremely detrimental to the rest of the system. Perhaps when we shift to the MVStore engine, we can revisit this design decision (using the StreamStore, that is, no connection at all).

Locking

Normally, the locking order in H2 is: first lock the Session object, then lock the Database object. However, in the case of the LOB data, we are using the system session to store the data. If we locked the normal way, we see deadlocks caused by the following pattern:

  Thread 1:
     locks normal session
     locks database
     waiting to lock system session
  Thread 2:
      locks system session
      waiting to lock database.
 

So, in this class alone, we do two things: we have our very own dedicated session, the LOB session, and we take the locks in this order: first the Database object, and then the LOB session. Since we own the LOB session, no-one else can lock on it, and we are safe.

嵌套类概要

嵌套类

限定符和类型	类和说明
class	LobStorageBackend.LobInputStream An input stream that reads from a LOB.

字段概要

字段

限定符和类型	字段和说明
static String	LOB_DATA_TABLE The name of the lob data table.

构造器概要

构造器

构造器和说明
LobStorageBackend(Database database)

方法概要

限定符和类型	方法和说明
ValueLobDb	copyLob(ValueLobDb old, int tableId, long length) Copy a lob.
Value	createBlob(InputStream in, long maxLength) Create a BLOB object.
Value	createClob(Reader reader, long maxLength) Create a CLOB object.
InputStream	getInputStream(ValueLobDb lob, byte[] hmac, long byteCount) Get the input stream for the given lob.
void	init() Initialize the lob storage.
boolean	isReadOnly() Whether the storage is read-only
void	removeAllForTable(int tableId) Remove all LOBs for this table.
void	removeLob(ValueLobDb lob) Delete a LOB (from the database, if it is stored there).
void	setTable(ValueLobDb lob, int table) Set the table reference of this lob.

从类继承的方法 java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

字段详细资料

LOB_DATA_TABLE

public static final String LOB_DATA_TABLE

The name of the lob data table. If this table exists, then lob storage is used.

另请参阅:

常量字段值

构造器详细资料

LobStorageBackend

public LobStorageBackend(Database database)

方法详细资料

init

public void init()

从接口复制的说明: LobStorageInterface

Initialize the lob storage.

指定者:

init 在接口中 LobStorageInterface

removeAllForTable

public void removeAllForTable(int tableId)

从接口复制的说明: LobStorageInterface

Remove all LOBs for this table.

指定者:

removeAllForTable 在接口中 LobStorageInterface

参数:

tableId - the table id

removeLob

public void removeLob(ValueLobDb lob)

从接口复制的说明: LobStorageInterface

Delete a LOB (from the database, if it is stored there).

指定者:

removeLob 在接口中 LobStorageInterface

参数:

lob - the lob

getInputStream

public InputStream getInputStream(ValueLobDb lob,
                                  byte[] hmac,
                                  long byteCount)
                           throws IOException

从接口复制的说明: LobStorageInterface

Get the input stream for the given lob.

指定者:

getInputStream 在接口中 LobStorageInterface

参数:

lob - the lob id

hmac - the message authentication code (for remote input streams)

byteCount - the number of bytes to read, or -1 if not known

返回:

the stream

抛出:

IOException

isReadOnly

public boolean isReadOnly()

从接口复制的说明: LobStorageInterface

Whether the storage is read-only

指定者:

isReadOnly 在接口中 LobStorageInterface

返回:

true if yes

copyLob

public ValueLobDb copyLob(ValueLobDb old,
                          int tableId,
                          long length)

从接口复制的说明: LobStorageInterface

Copy a lob.

指定者:

copyLob 在接口中 LobStorageInterface

参数:

old - the old lob

tableId - the new table id

length - the length

返回:

the new lob

createBlob

public Value createBlob(InputStream in,
                        long maxLength)

从接口复制的说明: LobStorageInterface

Create a BLOB object.

指定者:

createBlob 在接口中 LobStorageInterface

参数:

in - the input stream

maxLength - the maximum length (-1 if not known)

返回:

the LOB

createClob

public Value createClob(Reader reader,
                        long maxLength)

从接口复制的说明: LobStorageInterface

Create a CLOB object.

指定者:

createClob 在接口中 LobStorageInterface

参数:

reader - the reader

maxLength - the maximum length (-1 if not known)

返回:

the LOB

setTable

public void setTable(ValueLobDb lob,
                     int table)

从接口复制的说明: LobStorageInterface

Set the table reference of this lob.

指定者:

setTable 在接口中 LobStorageInterface

参数:

lob - the lob

table - the table