Go语言视频零基础入门到精通

yy2011

一、被注解了的javaBean类

@DBTable(name = "MEMBER")
public class Member {
  @SQLString(30) String firstName;
  @SQLString(50) String lastName;
  @SQLInteger Integer age;
  @SQLString(value = 30, constraints = @Constraints(primaryKey = true))  String handle;
  static int memberCount;
  public String getHandle() { return handle; }
  public String getFirstName() { return firstName; }
  public String getLastName() { return lastName; }
  public String toString() { return handle; }
  public Integer getAge() { return age; }
} ///:~
二、自定义的注解
import java.lang.annotation.*;
//这个注解用于生成数据库表
@Target(ElementType.TYPE) // 该注解用于类
@Retention(RetentionPolicy.RUNTIME)//运行期也保留注解
public @interface DBTable {
  public String name() default "";
} ///:~
import java.lang.annotation.*;
//该注解用于修饰JavaBean的域
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLInteger {
  String name() default "";
  Constraints constraints() default @Constraints;
} ///:~
import java.lang.annotation.*;
//该注解用于修饰JavaBean的域
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLString {
  int value() default 0;
  String name() default "";
  Constraints constraints() default @Constraints;
} ///:~
import java.lang.annotation.*;
//该注解用于修饰JavaBean的域
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Constraints {
  boolean primaryKey() default false;
  boolean allowNull() default true;
  boolean unique() default false;
} ///:~
三、处理类
下面的程序,从命令行传入一个类名并读取类文件,检查其上的数据库注解,然后生成用来创建数据库表的SQL命令.
import java.lang.annotation.*;
import java.lang.reflect.*;
import java.util.*;
public class TableCreator {
  public static void main(String[] args) throws Exception {
    if(args.length < 1) {
      System.out.println("arguments: annotated classes");
      System.exit(0);
    }
    for(String className : args) {
      Class< ? > cl = Class.forName(className);
      DBTable dbTable = cl.getAnnotation(DBTable.class);
      if(dbTable == null) {
        System.out.println(
          "No DBTable annotations in class " + className);
        continue;
      }
      String tableName = dbTable.name();
      // If the name is empty, use the Class name:
      if(tableName.length() < 1)
        tableName = cl.getName().toUpperCase();
      List columnDefs = new ArrayList();
      for(Field field : cl.getDeclaredFields()) {
        String columnName = null;
        Annotation[] anns = field.getDeclaredAnnotations();
        if(anns.length < 1)
          continue; // Not a db table column
        if(anns[0] instanceof SQLInteger) {
          SQLInteger sInt = (SQLInteger) anns[0];
          // Use field name if name not specified
          if(sInt.name().length() < 1)
            columnName = field.getName().toUpperCase();
          else
            columnName = sInt.name();
          columnDefs.add(columnName + " INT" +
            getConstraints(sInt.constraints()));
        }
        if(anns[0] instanceof SQLString) {
          SQLString sString = (SQLString) anns[0];
          // Use field name if name not specified.
          if(sString.name().length() < 1)
            columnName = field.getName().toUpperCase();
          else
            columnName = sString.name();
          columnDefs.add(columnName + " VARCHAR(" +
            sString.value() + ")" +
            getConstraints(sString.constraints()));
        }
        StringBuilder createCommand = new StringBuilder(
          "CREATE TABLE " + tableName + "(");
        for(String columnDef : columnDefs)
          createCommand.append("\n    " + columnDef + ",");
        // Remove trailing comma
        String tableCreate = createCommand.substring(
          0, createCommand.length() - 1) + ");";
        System.out.println("Table Creation SQL for " +
          className + " is :\n" + tableCreate);
      }
    }
  }
  private static String getConstraints(Constraints con) {
    String constraints = "";
    if(!con.allowNull())
      constraints += " NOT NULL";
    if(con.primaryKey())
      constraints += " PRIMARY KEY";
    if(con.unique())
      constraints += " UNIQUE";
    return constraints;
  }
}
/* 运行结果:
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30));
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50));
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50),
    AGE INT);
Table Creation SQL for annotations.database.Member is :
CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50),
    AGE INT,
    HANDLE VARCHAR(30) PRIMARY KEY);
*///:~

gnwe1990