Jpa 重写方言dialect 使用oracle / mysql 数据库自定义函数

在使用criteria api进行查询时 criteriaBuilder只提供了一个部分标准的sql函数，但当我们需要使用oracle特有的行转列函数wm_concat或

mysql特有的行转列函数group_concat时，就只能自己封装了，

criteriaBuilder提供了一个封装函数的方法：

    /**
     * Create an expression for the execution of a database
     * function.
     * @param name  function name
     * @param type  expected result type
     * @param args  function arguments
     * @return expression
     */
   <T> Expression<T> function(String name, Class<T> type,
Expression<?>... args);

如我们封装wm_concat函数，代码如下：

Expression<String> wmConcat = cb.function("wm_concat",
                String.class, root.get("ID"));

生成的sql 如：select  wm_concat(id)........

如果我们想生成 select wm_concat(distinct(id))这个的形式，来去id进行去重distinct。可以用下面的代码，对function进行嵌套：

        Expression<String> distinct = cb.function("distinct",
                String.class, root.get("ID"));

        Expression<String> wmConcat = cb.function("wm_concat",
                String.class, distinct);

当然这还没有完，生成这样的函数后，其实jpa是不认可wm_concat函数的,(distinct函数，lower函数等是认可的)，这是为什么呢，因为jpa认可的oracle函数都会在OracleDialect定义。

如我使用的OracleDialect版本是Oracle12cDialect，它本身没有注册自定义函数，但是它继承的类Oracle8iDialect注册了很多函数，如下：

protected void registerFunctions() {
        registerFunction( "abs", new StandardSQLFunction("abs") );
        registerFunction( "sign", new StandardSQLFunction("sign", StandardBasicTypes.INTEGER) );

        registerFunction( "acos", new StandardSQLFunction("acos", StandardBasicTypes.DOUBLE) );
        registerFunction( "asin", new StandardSQLFunction("asin", StandardBasicTypes.DOUBLE) );
        registerFunction( "atan", new StandardSQLFunction("atan", StandardBasicTypes.DOUBLE) );
        registerFunction( "bitand", new StandardSQLFunction("bitand") );
        registerFunction( "cos", new StandardSQLFunction("cos", StandardBasicTypes.DOUBLE) );
        registerFunction( "cosh", new StandardSQLFunction("cosh", StandardBasicTypes.DOUBLE) );
        registerFunction( "exp", new StandardSQLFunction("exp", StandardBasicTypes.DOUBLE) );
        registerFunction( "ln", new StandardSQLFunction("ln", StandardBasicTypes.DOUBLE) );
        registerFunction( "sin", new StandardSQLFunction("sin", StandardBasicTypes.DOUBLE) );
        registerFunction( "sinh", new StandardSQLFunction("sinh", StandardBasicTypes.DOUBLE) );
        registerFunction( "stddev", new StandardSQLFunction("stddev", StandardBasicTypes.DOUBLE) );
        registerFunction( "sqrt", new StandardSQLFunction("sqrt", StandardBasicTypes.DOUBLE) );
        registerFunction( "tan", new StandardSQLFunction("tan", StandardBasicTypes.DOUBLE) );
        registerFunction( "tanh", new StandardSQLFunction("tanh", StandardBasicTypes.DOUBLE) );
        registerFunction( "variance", new StandardSQLFunction("variance", StandardBasicTypes.DOUBLE) );

        registerFunction( "round", new StandardSQLFunction("round") );
        registerFunction( "trunc", new StandardSQLFunction("trunc") );
        registerFunction( "ceil", new StandardSQLFunction("ceil") );
        registerFunction( "floor", new StandardSQLFunction("floor") );

        registerFunction( "chr", new StandardSQLFunction("chr", StandardBasicTypes.CHARACTER) );
        registerFunction( "initcap", new StandardSQLFunction("initcap") );
        registerFunction( "lower", new StandardSQLFunction("lower") );
        registerFunction( "ltrim", new StandardSQLFunction("ltrim") );
        registerFunction( "rtrim", new StandardSQLFunction("rtrim") );
        registerFunction( "soundex", new StandardSQLFunction("soundex") );
        registerFunction( "upper", new StandardSQLFunction("upper") );
        registerFunction( "ascii", new StandardSQLFunction("ascii", StandardBasicTypes.INTEGER) );

        registerFunction( "to_char", new StandardSQLFunction("to_char", StandardBasicTypes.STRING) );
        registerFunction( "to_date", new StandardSQLFunction("to_date", StandardBasicTypes.TIMESTAMP) );

        registerFunction( "current_date", new NoArgSQLFunction("current_date", StandardBasicTypes.DATE, false) );
        registerFunction( "current_time", new NoArgSQLFunction("current_timestamp", StandardBasicTypes.TIME, false) );
        registerFunction( "current_timestamp", new NoArgSQLFunction("current_timestamp", StandardBasicTypes.TIMESTAMP, false) );

        registerFunction( "last_day", new StandardSQLFunction("last_day", StandardBasicTypes.DATE) );
        registerFunction( "sysdate", new NoArgSQLFunction("sysdate", StandardBasicTypes.DATE, false) );
        registerFunction( "systimestamp", new NoArgSQLFunction("systimestamp", StandardBasicTypes.TIMESTAMP, false) );
        registerFunction( "uid", new NoArgSQLFunction("uid", StandardBasicTypes.INTEGER, false) );
        registerFunction( "user", new NoArgSQLFunction("user", StandardBasicTypes.STRING, false) );

        registerFunction( "rowid", new NoArgSQLFunction("rowid", StandardBasicTypes.LONG, false) );
        registerFunction( "rownum", new NoArgSQLFunction("rownum", StandardBasicTypes.LONG, false) );

        // Multi-param string dialect functions...
        registerFunction( "concat", new VarArgsSQLFunction(StandardBasicTypes.STRING, "", "||", "") );
        registerFunction( "instr", new StandardSQLFunction("instr", StandardBasicTypes.INTEGER) );
        registerFunction( "instrb", new StandardSQLFunction("instrb", StandardBasicTypes.INTEGER) );
        registerFunction( "lpad", new StandardSQLFunction("lpad", StandardBasicTypes.STRING) );
        registerFunction( "replace", new StandardSQLFunction("replace", StandardBasicTypes.STRING) );
        registerFunction( "rpad", new StandardSQLFunction("rpad", StandardBasicTypes.STRING) );
        registerFunction( "substr", new StandardSQLFunction("substr", StandardBasicTypes.STRING) );
        registerFunction( "substrb", new StandardSQLFunction("substrb", StandardBasicTypes.STRING) );
        registerFunction( "translate", new StandardSQLFunction("translate", StandardBasicTypes.STRING) );

        registerFunction( "substring", new StandardSQLFunction( "substr", StandardBasicTypes.STRING ) );
        registerFunction( "locate", new SQLFunctionTemplate( StandardBasicTypes.INTEGER, "instr(?2,?1)" ) );
        registerFunction( "bit_length", new SQLFunctionTemplate( StandardBasicTypes.INTEGER, "vsize(?1)*8" ) );
        registerFunction( "coalesce", new NvlFunction() );

        // Multi-param numeric dialect functions...
        registerFunction( "atan2", new StandardSQLFunction("atan2", StandardBasicTypes.FLOAT) );
        registerFunction( "log", new StandardSQLFunction("log", StandardBasicTypes.INTEGER) );
        registerFunction( "mod", new StandardSQLFunction("mod", StandardBasicTypes.INTEGER) );
        registerFunction( "nvl", new StandardSQLFunction("nvl") );
        registerFunction( "nvl2", new StandardSQLFunction("nvl2") );
        registerFunction( "power", new StandardSQLFunction("power", StandardBasicTypes.FLOAT) );

        // Multi-param date dialect functions...
        registerFunction( "add_months", new StandardSQLFunction("add_months", StandardBasicTypes.DATE) );
        registerFunction( "months_between", new StandardSQLFunction("months_between", StandardBasicTypes.FLOAT) );
        registerFunction( "next_day", new StandardSQLFunction("next_day", StandardBasicTypes.DATE) );

        registerFunction( "str", new StandardSQLFunction("to_char", StandardBasicTypes.STRING) );
    }

通过调试代码，可以看到oracledialect共注册了80个函数。

同理，我们也可继承Oracle12cDialect，添加自己的函数：

public class MyOracleDialect extends Oracle12cDialect {

    /**
     * 添加oracler内置函数和自定义函数
     */
    @Override
    protected void registerFunctions() {
        super.registerFunctions();
        registerFunction("wm_concat", new StandardSQLFunction("wm_concat", StandardBasicTypes.STRING));
    }

    /**
     * 解决启动时数字溢出的错误
     * @return
     */
    @Override
    public String getQuerySequencesString() {
        return "select * from user_sequences";
    }
}

配置完成后，启动项目即可。

原文地址：https://www.cnblogs.com/hankuikui/p/11734194.html