使用自定义注解搭建简单框架

本文主要介绍如何使用Java运行时级别的注解配合反射来搭建框架，以下是构建sql表的简单例子，阐明了搭建运行时框架的方法


1.定义两个运行时注解，分别作用于类和字段表示表名和列名

@Retention(RetentionPolicy.RUNTIME)
public @interface Table {
	String name();
}

@Retention(RetentionPolicy.RUNTIME)
public @interface Column {
    String name(); 
}

2.定义存储列的模型NameAndType

public class NameAndType {

	private String name;
	
	private String type;
	
	public NameAndType() 
	{
	}
	
	public NameAndType(String type, String name) 
	{
		this.type = type;
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getType() {
		return type;
	}

	public void setType(String type) {
		this.type = type;
	}
}

3.定义javabea 和数据库之间的关系映射

@Table(name="BeanTable")
public class Bean {

	 @Column(name="field") //注解@Colomn 具体实现接下来会讲
	 int field;
	 @Column(name="description")
	 String description;
}

4.通过反射定义工具类，定义动态获取表名，获取列相关信息和组装SQL

public class Utils {
	private static String getTableName(Class<?> clazz)
	{
		String name = null;
		if(clazz.isAnnotationPresent(Table.class))
		{
			Table table = clazz.getAnnotation(Table.class);
			name = table.name();
		}
		return name;
	}
	private static List<NameAndType> getColumns(Class<?> clazz)
	{
		List<NameAndType> list = new ArrayList<>();
		Field[] fields = clazz.getDeclaredFields();
		if(fields != null)
		{
			for(Field field : fields)
			{
				if(field.isAnnotationPresent(Column.class))
				{
					Column column = field.getAnnotation(Column.class);
					String name = column.name();
				    String type = null;
				    if(int.class.isAssignableFrom(field.getType()))
				    {
				    	type = "integer";
				    }
				    else if(String.class.isAssignableFrom(field.getType()))
				    {
				    	type = "text";
				    }
				    else 
				    {
				    	throw new RuntimeException("unsupport type :" + field.getType().getSimpleName());
				    }
				    
				    list.add(new NameAndType(type, name));
					
				}
			}
		}
		return list;
	}
	
	
	public static String createTable(Class<?> bean) {
	    String tableName = getTableName(bean);
	    List<NameAndType> columns = getColumns(bean);
	    if (tableName != null && !tableName.equals("") && !columns.isEmpty()) {
	        StringBuilder createTableSql = new StringBuilder("create table ");
	        //加表名
	        createTableSql.append(tableName);
	        createTableSql.append("(");

	        //加表中字段
	        for (int i = 0; i < columns.size(); i++) {
	            NameAndType column = columns.get(i);
	            createTableSql.append(column.getName());
	            createTableSql.append(" ");
	            createTableSql.append(column.getType());
	            // 追加下一个字段定义前需要添加逗号
	            if (i != columns.size() - 1) {
	                createTableSql.append(",");
	            }
	        }
	        createTableSql.append(")");
	        return createTableSql.toString();
	    }

	    return null;
	}
}

public class Test {

	public static void main(String[] args) 
	{
		 System.out.println(Utils.createTable(Bean.class));
	}
}

---运行结果如下：

create table BeanTable(field integer,description text)