How to specify Double’s precision on hibernate?

The length element of the Column annotation applies only if a string-valued column is used. In your case, you should use the precision and the scale elements.

@Column(precision=10, scale=2)

Here is what the specification writes about them:

• int – precision – (Optional) The precision for a decimal (exact
  numeric) column. (Applies only if a
  decimal column is used.)
• int – scale – (Optional) The scale for a decimal (exact numeric)
  column. (Applies only if a decimal
  column is used.)

References

• JPA 1.0 Specification
  • Section 9.1.5 “Column Annotation”