休眠如何生成外键约束名称？

Question

hibernate如何生成外键约束名称？

如果我没有定义名称，hibernate将生成类似这样的内容

CONSTRAINT fk_2ocepcfwpr1v18dg1ieoe6bau

这个名字是怎么产生的？也许来自MD5字段名称的哈希值或类似的东西？我需要知道所有实例上的名称是否相等。

Answer 1

Hibernate通过串联表和属性名称来生成约束名称，并将结果转换为MD5。由于某些数据库中的约束名称长度限制，因此需要它。例如，在Oracle数据库中，外键名称的长度不能超过30个符号长度。

Hibernate源org.hibernate.mapping.Constraint中的此代码段

/**
 * If a constraint is not explicitly named, this is called to generate
 * a unique hash using the table and column names.
 * Static so the name can be generated prior to creating the Constraint.
 * They're cached, keyed by name, in multiple locations.
 *
 * @return String The generated name
 */
public static String generateName(String prefix, Table table, Column... columns) {
    // Use a concatenation that guarantees uniqueness, even if identical names
    // exist between all table and column identifiers.

    StringBuilder sb = new StringBuilder( "table`" + table.getName() + "`" );

    // Ensure a consistent ordering of columns, regardless of the order
    // they were bound.
    // Clone the list, as sometimes a set of order-dependent Column
    // bindings are given.
    Column[] alphabeticalColumns = columns.clone();
    Arrays.sort( alphabeticalColumns, ColumnComparator.INSTANCE );
    for ( Column column : alphabeticalColumns ) {
        String columnName = column == null ? "" : column.getName();
        sb.append( "column`" ).append( columnName ).append( "`" );
    }
    return prefix + hashedName( sb.toString() );
}

/**
 * Hash a constraint name using MD5. Convert the MD5 digest to base 35
 * (full alphanumeric), guaranteeing
 * that the length of the name will always be smaller than the 30
 * character identifier restriction enforced by a few dialects.
 * 
 * @param s
 *            The name to be hashed.
 * @return String The hased name.
 */
public static String hashedName(String s) {
    try {
        MessageDigest md = MessageDigest.getInstance( "MD5" );
        md.reset();
        md.update( s.getBytes() );
        byte[] digest = md.digest();
        BigInteger bigInt = new BigInteger( 1, digest );
        // By converting to base 35 (full alphanumeric), we guarantee
        // that the length of the name will always be smaller than the 30
        // character identifier restriction enforced by a few dialects.
        return bigInt.toString( 35 );
    }
    catch ( NoSuchAlgorithmException e ) {
        throw new HibernateException( "Unable to generate a hashed Constraint name!", e );
    }
}

您可以使用生成自己的约束名称（唯一键和外键）ImplicitNamingStrategy。您可以参考Hibernate5NamingStrategy作为示例。