Jsqlparser series of two code structure (original)

Jsqlparser series of two code structure (original)

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The previous article briefly describes how to set up the Jsqlparser project, and this chapter introduces the code structure of the Jsqlparser project in general.

I. Directory structure

Jsqlparser directory structure is relatively simple, mainly have expressions, parsers, statements processing several directories. The following diagram shows a specific SQL statement and an expression:

Next, briefly describe some of the abstract concepts in Jsqlparser.

II. SQL statements (statement)

Jsqlpaser all SQL statements as statement,statement represents an operation on the database.

 Package net.sf.jsqlparser.statement; /**  */Publicinterface  Statement    {void  Accept ( Statementvisitor statementvisitor);}

Common statement are:select,create,Drop,Insert,Delete , etc., which are implemented as statement classes, All implement the Accept method. This is a typical application of the visitor pattern, running through the Jsqlparser parsing the SQL statements in every corner. Here you just need to know statement corresponds to Statementvisitor. If you want to customize the SQL statement, you only need to implement the Statementvisitor interface.

In the case of a query statement (select), here is a method for the Select class:

As you can see, the Select object has two important members:selectbody, List<withitem>, where Withitem corresponds to the WITH keyword of the SQL statement and is rare. The focus of common SELECT statements is visible in Selectbody. Selectbody is an interface that is defined as follows:

 Package Net.sf.jsqlparser.statement.select;  Public Interface selectbody {    void  Accept (Selectvisitor selectvisitor);}

Again a visitor, but here replaced by Selectvisitor. That is, the visitor for the SELECT statement. If you want to customize the parse SELECT statement, you can implement the interface.

The SELECT statement is further subdivided and can be broadly expressed as follows:

Select   SelectItem from Fromitem where Expression

Where SelectItem represents the content to select:

/***/Publicinterface  selectitem    {void  Accept ( Selectitemvisitor selectitemvisitor);}

Fromitem represents the data source (table or embedded selection statement)

 package   Net.sf.jsqlparser.statement.select;  import   Net.sf.jsqlparser.expression.Alias;  /**   * an item in a "SELECT [...] From Item1 "statement. (For example a table or a * sub-select)  */ public  interface   Fromitem {  accept (Fromitemvisitor fromitemvisitor);    Alias Getalias ();     void   Setalias (alias alias);    Pivot Getpivot ();  void   Setpivot (pivot pivot);}  

You can see that SelectItem parsing will be used when the Seletitemvisitor,fromitem parsing will use the Fromitemvisitor, the pattern is the same.
Expression is slightly more complex and is described separately below.

Iii. Expressions (expression)

During SQL parsing, the parsing of conditions is the most complex. Jsqlparser abstract The conditional expression between where and order by exception.

 Package net.sf.jsqlparser.expression;  Public Interface Expression {    void  Accept (ExpressionVisitor expressionvisitor);}

Haha, also see a expressionvisitor, is not think Jsqlparser idea is quite concise.
Expression is further subdivided into a variety of more common:

1. Conditional expressions

such as: Andexpression (and), orexpression (or)

2. Relationship Expression

such as: equalsto (=), Minorthan (<), GreaterThan (>), ...

3. Arithmetic expressions

such as: addition (+), subtraction (-), multiplication (*), Division (/),......

4. Column expression

such as: Column

5. Case expression

such as: caseexpression

6. Value Expression

such as: Stringvalue,datevalue,longvalue,doublevalue,......

7. Function expression

such as: Function

8. Parameter expression

such as: Jdbcparameter,jdbcnameparameter,......

If you want to customize the ExpressionVisitor, for the above different expressions, you should give corresponding processing.

Iv. Visitors (Visitor)

As mentioned above, commonly used Visitor have statementvisitor,selectvisitor,Expressionvisitor,selectitemvisitor,fromitemvisitor, etc. , it is necessary to customize the relevant visitor when parsing a specific piece of SQL statement.

Jsqlparser series of two code structure (original)