LockModeType (Java EE 5 SDK)

javax.persistence Enum LockModeType

java.lang.Object
  java.lang.Enum<LockModeType>
      javax.persistence.LockModeType

所有已实现的接口:

Serializable, Comparable<LockModeType>

public enum LockModeType

extends Enum<LockModeType>

Lock modes that can be specified by means of the EntityManager.lock() method.

The semantics of requesting locks of type LockModeType.READ and LockModeType.WRITE are the following.

If transaction T1 calls lock(entity, LockModeType.READ) on a versioned object, the entity manager must ensure that neither of the following phenomena can occur:

• P1 (Dirty read): Transaction T1 modifies a row. Another transaction T2 then reads that row and obtains the modified value, before T1 has committed or rolled back. Transaction T2 eventually commits successfully; it does not matter whether T1 commits or rolls back and whether it does so before or after T2 commits.
P2 (Non-repeatable read): Transaction T1 reads a row. Another transaction T2 then modifies or deletes that row, before T1 has committed. Both transactions eventually commit successfully.

Lock modes must always prevent the phenomena P1 and P2.

In addition, calling lock(entity, LockModeType.WRITE) on a versioned object, will also force an update (increment) to the entity's version column.

The persistence implementation is not required to support calling EntityManager.lock() on a non-versioned object. When it cannot support a such lock call, it must throw the PersistenceException.

从以下版本开始:

Java Persistence 1.0

READ

public static final LockModeType READ

Read lock

WRITE

public static final LockModeType WRITE

Write lock

values

public static final LockModeType[] values()

Returns an array containing the constants of this enum type, in the order they're declared. This method may be used to iterate over the constants as follows:

for(LockModeType c : LockModeType.values())
        System.out.println(c);

返回:

an array containing the constants of this enum type, in the order they're declared

valueOf

public static LockModeType valueOf(String name)

Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)

参数:

name - the name of the enum constant to be returned.

返回:

the enum constant with the specified name

抛出异常:

IllegalArgumentException - if this enum type has no constant with the specified name

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