java语言

2021-07-30

注解

一个例子

// UseCase.java
package test;

import java.lang.annotation.*;

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface UseCase {
	public int id();
	public String description() default "no description";
}

// PasswordUtils.java
package test;

import java.util.*;

public class PasswordUtils {
	@UseCase(id=47, description=
			"Password must contain at least one numeric")
	public boolean validatePassword(String password) {
		return (password.matches("\\w*\\d\\w*"));
	}
	@UseCase(id=48)
	public String encryptPassword(String password) {
		return new StringBuilder(password).reverse().toString();
	}
	@UseCase(id=49, description=
			"New password can not equal previously used ones")
	public boolean checkForNewPassword(
			List<String> prevPasswords, String password) {
		return !prevPasswords.contains(password);
	}
}

// UseCaseTracker.java
package test;

import java.lang.reflect.*;
import java.util.*;

// 注解处理器
public class UseCaseTracker {
	public static void trackUseCases(
			List<Integer> useCases, Class<?> cl) {
		for(Method m : cl.getDeclaredMethods()) {
			UseCase uc = m.getAnnotation(UseCase.class);
			if(uc != null) {
				System.out.println("Found use case: " + uc.id() +
						" " + uc.description());
				useCases.remove(new Integer(uc.id()));
			}
		}
		for(int i : useCases) {
			System.out.println("Warning: missing use case-" + i);
		}
	}
	public static void main(String[] args) {
		List<Integer> useCases = new ArrayList<Integer>();
		Collections.addAll(useCases, 47, 48, 49, 50);
        // PasswordUtils类中实现了很多用例，但是不知道有没有覆盖useCases中的全部
        // 由于每个函数都通过注解@UseCase标注使用了什么用例，
        // 因此通过trackUseCases函数来分析PasswordUtils类中哪些用例没有实现
		trackUseCases(useCases, PasswordUtils.class);
	}
}

如果不使用注解处理器，编写和使用注解不会比注释更有用。

编写注解处理器

为一个bean添加很多自定义注解，然后编写注解处理器，处理这些注解。（据我所知，一个bean是不是一个数据库表的定义啊）

// DBTable.java
package annotations.database;
import java.lang.annotation.*;

@Target(ElementType.TYPE) // applies to classes only
@Retention(RetentionPolicy.RUNTIME)
public @interface DBTable {
	public String name() default "";
}

// Constraints.java
package annotations.database;
import java.lang.annotation.*;

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

// SQLString.java
package annotations.database;
import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLString {
	int value() default 0;
	String name() default "";
	Constraints constraints() default @Constraints;
}

// SQLInteger.java
package annotations.database;
import java.lang.annotation.*;

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface SQLInteger {
	String name() default "";
	Constraints constraints() default @Constraints;
}

// Member.java
package annotations.database;

@DBTable(name = "MEMBER")
public class Member {
	@SQLString(30) String firstName; // 默认给value属性设置值，而且必须是value属性
	@SQLString(50) String lastName;
	@SQLInteger Integer age;
	@SQLString(value=30,
			constraints=@Constraints(primaryKey=true))
	String handle;
	static int memberCount;
	public String getHandle() {
		return this.handle;
	}
	public String getFirstName() {
		return this.firstName;
	}
	public String getLastName() {
		return this.lastName;
	}
	public String toString() {
		return this.handle;
	}
	public Integer getAge() {
		return this.age;
	}
}

// TableCreate.java
// 注解解释器
package annotations.database;
import java.lang.annotation.*;
import java.util.*;
import java.lang.reflect.*;

public class TableCreator {
	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;
	}
	
	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(tableName.length() < 1) {
				tableName = cl.getName().toUpperCase();
			}
			List<String> columnDefs = new ArrayList<String>();
			for(Field field : cl.getDeclaredFields()) {
				String columnName = null;
				Annotation[] anns = field.getDeclaredAnnotations();
				if(anns.length < 1)
					continue;
				if(anns[0] instanceof SQLInteger) {
					SQLInteger sInt = (SQLInteger)anns[0];
					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];
					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 + ",");
			String tableCreate = createCommand.substring(
					0, createCommand.length()-1) + ");";
			System.out.println("Table Creation SQL for " + 
					className + " is: \n" + tableCreate);	
		}
	}
}

输入参数：annotations.database.Member

输出：

Table Creation SQL for annotations.database.Member is: 
CREATE TABLE MEMBER(
    FIRSTNAME VARCHAR(30),
    LASTNAME VARCHAR(50),
    AGE INT,
    HANDLE VARCHAR(30) PRIMARY KEY);

引用

官方文档：https://docs.oracle.com/en/java/javase/16/

jdk源码：https://github.com/openjdk/jdk 也可以在官方文档中找到