编写注解处理器拼接SQL

分手后的思念是犯贱 2022-06-06 08:55 237阅读 0赞

说明：本文将采用倒叙方式陈述注解处理器

1.先看结果，抛砖引玉

为annotations.javabean.Member创建的SQL： 
     CREATE TABLE MEMBER(
     FIRSTNAME VARCHAR(30) NOT NULL);
     
    为annotations.javabean.Member创建的SQL： 
     CREATE TABLE MEMBER(
     FIRSTNAME VARCHAR(30) NOT NULL,
     LASTNAME VARCHAR(50) NOT NULL);
     
    为annotations.javabean.Member创建的SQL： 
     CREATE TABLE MEMBER(
     FIRSTNAME VARCHAR(30) NOT NULL,
     LASTNAME VARCHAR(50) NOT NULL,
     AGE INT NOT NULL);
     
    为annotations.javabean.Member创建的SQL： 
     CREATE TABLE MEMBER(
     FIRSTNAME VARCHAR(30) NOT NULL,
     LASTNAME VARCHAR(50) NOT NULL,
     AGE INT NOT NULL,
     HANDLE VARCHAR(30) NOT NULL PRIMARY KEY);

2.注解处理器代码（运行时在IDE中进行参数配置对应javabean的全限定名）

package annotations.ImplAnnotation;
    
    import java.lang.annotation.Annotation;
    import java.lang.reflect.Field;
    import java.util.ArrayList;
    import java.util.List;
    
    import annotations.database.Constraints;
    import annotations.database.DBTable;
    import annotations.database.SQLInteger;
    import annotations.database.SQLString;
    
    public class TableCreater {
    	public static void main(String[] args) throws ClassNotFoundException {
    		if(args.length<1){
    			System.out.println("参数是注解类型");
    			System.exit(0);
    		}
    		for(String className:args){
    			//通过反射机制获取“全类名”对应的Class对象
    			Class<?> cl = Class.forName(className);
    			//获取DBTable注解
    			DBTable dbTable = cl.getAnnotation(DBTable.class);
    			//判断是类中是否有DBTable注解
    			if(dbTable == null){
    				System.out.println("类中没有DBTable注解："+className);
    				continue;
    			}
    			String tableName = dbTable.name();
    			/*
    			 * 如果DBTable注解存在,但长度小于1
    			 * （"".length()=0）,则取对应
    			 * 类名作为表名
    			 * */
    			if(tableName.length() < 1){
    				tableName = cl.getName().toUpperCase();
    			}
    			List<String> columnDefs = new ArrayList<String>();
    			/*
    			 *1.通过反射机制依次获取类中属性对应的注解
    			 *
    			 *3.
    			 * */
    			for (Field field:cl.getDeclaredFields()) {
    				String columnName = null;
    				Annotation[] anns = field.getDeclaredAnnotations();
    				if(anns.length < 1) continue; //not a db table column
    				//2.对获取到的注解进行instanceof判断
    				if(anns[0] instanceof SQLInteger){
    					SQLInteger sInt = (SQLInteger) anns[0];
    					//如果没有指定SQLInteger注解的name，则将类中属性名称作为数据库列名
    					if(sInt.name().length() < 1)
    						columnName = field.getName().toUpperCase();
    					else
    						columnName = sInt.name();
    					columnDefs.add(columnName+" INT"+getConstraints(sInt.constraints()));
    				}
    				//2.对获取到的注解进行instanceof判断
    				if(anns[0] instanceof SQLString){
    					SQLString sString = (SQLString) anns[0];
    					//空字符串 判断
    					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+"(");
    				String tableCreate = "";
    				for (String columnDef : columnDefs) {
    					createCommand.append("\n " + columnDef + ",");
    					tableCreate = createCommand.substring(
    							0,createCommand.length()-1)+");";
    				}
    				//循环输出每次SQL拼接结果
    				System.out.println("为"+className+"创建的SQL： "+
    						"\n "+ tableCreate+"\n ");
    			}
    		}
    	}
    	
    	/*
    	 * 获取约束的方法
    	 * */
    	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;
    	}
    	
    }

3.背注解的javabean

package annotations.javabean;
    
    import annotations.database.Constraints;
    import annotations.database.DBTable;
    import annotations.database.SQLInteger;
    import annotations.database.SQLString;
    
    @DBTable(name="MEMBER")//表名
    public class Member {
    	//长度30
    	@SQLString(30) 
    	String firstName;
    	
    	//长度50
    	@SQLString(50) String lastName;
    	
    	@SQLInteger Integer age;
    	
    	@SQLString(value=30,constraints=
    			@Constraints(primaryKey=true))
    	String handle;
    	
    	static int memberCount;
    
    	public String getFirstName() {
    		return firstName;
    	}
    
    	public String getLastName() {
    		return lastName;
    	}
    
    	public Integer getAge() {
    		return age;
    	}
    
    	public String getHandle() {
    		return handle;
    	}
    	
    	@Override
    	public String toString(){
    		return handle;
    	}
    	
    }

4.注解类

4.1注解类DBTable

package annotations.database;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.TYPE)//应用目标：接口、类、枚举、注解
    @Retention(RetentionPolicy.RUNTIME)//保留策略：运行时
    public @interface DBTable {//数据库注解
    	public String name() default "";
    }

4.2注解类Constraints

package annotations.database;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.FIELD)//应用目标:字段、枚举的常量
    @Retention(RetentionPolicy.RUNTIME)//保留策略：运行时
    public @interface Constraints {//约束注解
    	boolean primaryKey() default false;
    	boolean allowNull() default false;
    	boolean unique() default false;
    }

4.3注解类Uniqueness

package annotations.database;
    
    public @interface Uniqueness {
    	Constraints constraints()
    		default @Constraints(unique=true);
    }

4.4注解类SQLString

package annotations.database;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.FIELD)//应用目标:字段、枚举的常量
    @Retention(RetentionPolicy.RUNTIME)//保留策略：运行时
    public @interface SQLString {
    	int value() default 0;
    	String name() default "";
    	/*
    	 * 注解类型的...
    	 * */
    	Constraints constraints() default @Constraints;
    }

4.5注解类SQLInteger

package annotations.database;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target(ElementType.FIELD)//应用目标:字段、枚举的常量
    @Retention(RetentionPolicy.RUNTIME)//保留策略：运行时
    public @interface SQLInteger {
    	String name() default "";
    	//注解类型的...
    	Constraints constraints() default @Constraints;
    }