Java注解annotation用法和自定义注解处理器

　　实体使用注解：这里是运用了运行时候处理注解，所以RetentionPolicy.RUNTIME

package whut.annotationDB;
@DBTable(name="MEMBER")
public class Member {
    //在使用注解过程中，如果有元素是value，并且只有value需要赋值，
    //则只需要在()中将值写入
    @SQLString(30)
    private String firstName;
    @SQLString(50)
    private String lastName;
    @SQLInteger
    private Integer age;
    @SQLString(value=30，constraints=@Constraints(primaryKey=true))
    private String handle;
    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public String getHandle() {
        return handle;
    }
    public void setHandle(String handle) {
        this.handle = handle;
    }
}

　　具体的非apt实现的注解处理器：

package whut.annotationDB;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class TableCreator {
    public Connection getConnection() {
        String user = "root";
        String password = "";
        String serverUrl = "jdbc:mysql://localhost:3306/carrent?user=root&password=";
        try {
            Class.forName("com.mysql.jdbc.Driver");
            Connection con = DriverManager.getConnection(serverUrl， user，
                    password);
            return con;
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
    //实现创建表
    public static void main(String[] args) {
        TableCreator tc = new TableCreator();
        tc.executeCreateDB(Member.class);
    }
    public void executeCreateDB(Class<?> entity) {
        String sqlStr = explainAnnotation(entity);
        Connection con = getConnection();
        PreparedStatement psql = null;
        if (con != null && !sqlStr.equals("error")) {
            try {
                psql = con.prepareStatement(sqlStr);
                psql.execute();
            } catch (SQLException e) {
                e.printStackTrace();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    if (psql != null)
                        psql.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
                try {
                    if (con != null)
                        psql.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        } else
            System.out.println("failure to...");
    }
    // 真正的处理器，Class<?>必须用这个表明
    public String explainAnnotation(Class<?> entity) {
        // 获取指定类型的注解
        DBTable dbtable = entity.getAnnotation(DBTable.class);
        if (dbtable == null) {
            System.out.println("No DBTable annotation in class"
                    + entity.getName());
            return "error";
        } else {
            String tableName = dbtable.name();// 获取注解name值，即表名称
            // 当没有设置name值，直接利用类的名作为表名
            if (tableName.length() < 1)
                tableName = entity.getName().toUpperCase();// 转换大写
            // 准备处理字段注解
            List<String> columnsDefs = new ArrayList<String>();
            // 获取该类的所有字段
            for (Field field : entity.getDeclaredFields()) {
                String columnName = null;
                // 获取该字段所有的注解
                Annotation[] anns = field.getDeclaredAnnotations();
                // Annotation[] anns=field.getAnnotations();
                // 当有注解的时候
                if (anns.length >= 1) {
                    // 判断注解的类型
                    if (anns[0] instanceof SQLInteger) {
                        SQLInteger sInt = (SQLInteger) anns[0];
                        // 当没有name时候，将字段大写为列名
                        if (sInt.name().length() < 1)
                            columnName = field.getName().toUpperCase();
                        else
                            columnName = sInt.name();
                        columnsDefs.add(columnName + " INT"
                                + getConstraints(sInt.constraints()));
                    }
                    if (anns[0] instanceof SQLString) {
                        SQLString sString = (SQLString) anns[0];
                        // 当没有name时候，将字段大写为列名
                        if (sString.name().length() < 1)
                            columnName = field.getName().toUpperCase();
                        else
                            columnName = sString.name();
                        columnsDefs.add(columnName + " VARCHAR("
                                + sString.value() + ")"
                                + getConstraints(sString.constraints()));
                    }
                }
            }
            StringBuilder createDB = new StringBuilder("CREATE TABLE "
                    + tableName + "(");
            for (String cols : columnsDefs)
                createDB.append(" " + cols + "，");
            // 移除后一个，号
            String tableSQL = createDB.substring(0， createDB.length() - 1)
                    + ");";
            // 输出创建表的过程
            System.out.println("Table Creation SQL is: " + tableSQL);
            return tableSQL;
        }
    }
    // 返回指定的约束
    public String getConstraints(Constraints con) {
        String constras = "";
        if (!con.allowNull())
            constras += " NOT NULL";
        if (con.primaryKey())
            constras += " PRIMARY KEY";
        if (con.unique())
            constras += " UNIQUE";
        return constras;
    }
}