最近在学习Spring源码的时候发现有个工具写的蛮不错的，所以分享出来

在AbstractPlatformTransactionManager类中，使用一个String类型的name获取对应常量的值

public final void setTransactionSynchronizationName(String constantName) { 		setTransactionSynchronization(constants.asNumber(constantName).intValue()); 	} 

源码学习

Constants是一个常量获取工具，在org.springframework.core包中

This class can be used to parse other classes containing constant definitions

in public static final members. The {@code asXXXX} methods of this class

allow these constant values to be accessed via their string names.

构造函数：通过反射的方式获取目标source类中所有的public static final的常量放入一个Map中

public Constants(Class<?> clazz) { 		Assert.notNull(clazz, "Class must not be null"); 		this.className = clazz.getName(); 		Field[] fields = clazz.getFields(); 		for (Field field : fields) { 			if (ReflectionUtils.isPublicStaticFinal(field)) { 				String name = field.getName(); 				try { 					Object value = field.get(null); 					this.fieldCache.put(name, value); 				} 				catch (IllegalAccessException ex) { 					// just leave this field and continue 				} 			} 		} 	} 

将获取的常量存放在一个Map中

/** The name of the introspected class */ 	private final String className;  	/** Map from String field name to object value */ 	private final Map<String, Object> fieldCache = new HashMap<>(); 

常用的几个方法：通过asXX方法取出相应的值

/** 	 * Return a constant value cast to a Number. 	 * @param code the name of the field (never {@code null}) 	 * @return the Number value 	 * @see #asObject 	 * @throws ConstantException if the field name wasn't found 	 * or if the type wasn't compatible with Number 	 */ 	public Number asNumber(String code) throws ConstantException { 		Object obj = asObject(code); 		if (!(obj instanceof Number)) { 			throw new ConstantException(this.className, code, "not a Number"); 		} 		return (Number) obj; 	}  	/** 	 * Return a constant value as a String. 	 * @param code the name of the field (never {@code null}) 	 * @return the String value 	 * Works even if it's not a string (invokes {@code toString()}). 	 * @see #asObject 	 * @throws ConstantException if the field name wasn't found 	 */ 	public String asString(String code) throws ConstantException { 		return asObject(code).toString(); 	}  	/** 	 * Parse the given String (upper or lower case accepted) and return 	 * the appropriate value if it's the name of a constant field in the 	 * class that we're analysing. 	 * @param code the name of the field (never {@code null}) 	 * @return the Object value 	 * @throws ConstantException if there's no such field 	 */ 	public Object asObject(String code) throws ConstantException { 		Assert.notNull(code, "Code must not be null"); 		String codeToUse = code.toUpperCase(Locale.ENGLISH); 		Object val = this.fieldCache.get(codeToUse); 		if (val == null) { 			throw new ConstantException(this.className, codeToUse, "not found"); 		} 		return val; 	}  

使用demo：

import org.springframework.core.Constants;  /**  * @Author: Kipeng Huang  * @Date: 2018-4-3 15:52  */ public class ConstantsLearn {     public static final int MAX_NUM = 5;     public static final int MIN_NUM = 2;     public static final String NAME = "kipeng";     public static void main(String[] args) {         Constants constants = new Constants(ConstantsLearn.class);         System.out.println("MAX_NUM:"+constants.asNumber("MAX_NUM").intValue());         System.out.println("NAME:"+constants.asString("NAME"));     }  } 

运行结果：

MAX_NUM:5 NAME:kipeng 

注意事项：

常量必须是 public static final 修饰的，否则使用asXX方法取出的时候抛exception

public ConstantException(String className, String field, String message) { 			super("Field '" + field + "' " + message + " in class [" + className + "]"); 		} 

反射的知识补充

在Constants的构造函数中使用了java反射的部分内容，所以补充下反射的部分知识

• getName 获取类名 包含了包名
• getSimpleName 获取简单的类名 不包含包名
• getFields 获取全部public 的参数
• getDeclaredFields 获得某个类的所有声明的字段，即包括public、private和proteced，但是不包括父类的申明字段
• getMethods 获取全部public 的methods
• getDeclaredMethods 获取全部的method including public, protected, default (package) access, and private methods, but excluding inherited methods