Java中的注解

Java中内置了三种标准注解：

@Override

@Deprecated

@SuppressWarnings

提供了四种源注解：

@Target

@Retention

@Documented
@Inherited

下边的例子通过注解实现了对象到数据表的映射，利用反射机制生成创建表的ＳＱＬ语句，实现ＯＲＭ

自定义注解：
package com.gbx.anotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/*
 * 定义关于约束的注解
 */

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Constraints {
	boolean primaryKey() default false;
	boolean allowNull() default false;
	boolean unique() default false;
}

package com.gbx.anotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/*
 * 定义关于数据库表的注解
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)

public @interface DBtable {
	String name() default "";
}


package com.gbx.anotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/*
 * 定义关于表中字段的注解
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLInteger {
	String name() default "";
	Constraints constraints() default @Constraints;
}

package com.gbx.anotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/*
 * 定义关于表中字段的注解
 */
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLString {
	int value() default 0;
	String name() default "";
	Constraints constraints() default @Constraints;
}

　　

对象：
@DBtable(name = "Memeber")
public class Member {
	@SQLString(30) //这里默认直接给vａｌｕｅ赋值， 
	public String firstName;
	@SQLString(50)
	public String lastName;
	@SQLInteger
	public Integer age;
	@SQLString(value = 30, constraints = @Constraints(primaryKey = true))
	public String handle;
	
	public int memberCount;
	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 int getMemberCount() {
		return memberCount;
	}
	public void setMemberCount(int memberCount) {
		this.memberCount = memberCount;
	}
}

　　

反射处理：
package com.gbx.bz;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import javax.jws.WebParam;

import com.gbx.anotation.Constraints;
import com.gbx.anotation.DBtable;
import com.gbx.anotation.SQLInteger;
import com.gbx.anotation.SQLString;
import com.gbx.po.Member;




public class Test{

	
	public static String getConstraints(Constraints constraints) {
		StringBuilder sb = new StringBuilder();
		if (constraints.primaryKey()) {
			sb.append(" PRIMARY KEY ");
		} 
		if (constraints.unique()) {
			sb.append(" UNIQUE ");
		}
		if (!constraints.allowNull()) {
			sb.append(" NOT NULL ");
		}
		return sb.toString();
	}
	/*
	 * 利用反射机制实现ORM...
	 */
	public static void main(String[] args) throws ClassNotFoundException {
		Class<?> cl = Class.forName(Member.class.getName());
		
		DBtable dbtable = cl.getAnnotation(DBtable.class);
		StringBuilder createSQL = new StringBuilder();
		if (dbtable == null) {
			System.out.println("没有DBTable的注解");
		} else {
			String tableName = dbtable.name();
			if (tableName == "") {
				tableName = cl.getSimpleName().toUpperCase();
			}
			createSQL.append("CREATE TABLE " + tableName + "(");
			
			for (Field field : cl.getFields()) {
				StringBuilder columInf = new StringBuilder();
				Annotation[] annotations = field.getAnnotations();
				if (annotations.length < 1) continue;
				String columName = null;
				if (annotations[0] instanceof SQLInteger) {
					SQLInteger sInt = (SQLInteger) annotations[0];
					columName = sInt.name();
					if (columName.length() < 1) {
						columName = field.getName().toUpperCase();
					}
					columInf.append(columName + " INT " + getConstraints(sInt.constraints()));
				} else if (annotations[0] instanceof SQLString) {
					SQLString sStr = (SQLString)annotations[0];
					columName = sStr.name();
					if (columName.length() < 1) {
						columName = field.getName().toUpperCase();
					}
					columInf.append(columName + " VARCHAR " + "(" + sStr.value() + ")" + " " + getConstraints(sStr.constraints()));
				}
				createSQL.append("
" + columInf.toString() + ",");
			}
			
			String createCommond = createSQL.substring(0, createSQL.length() - 1) + ");";
			System.out.println(createCommond);
		}
		
	}
}

　　

输出：
CREATE TABLE Memeber(
FIRSTNAME VARCHAR (30)  NOT NULL ,
LASTNAME VARCHAR (50)  NOT NULL ,
AGE INT  NOT NULL ,
HANDLE VARCHAR (30)  PRIMARY KEY  NOT NULL );