[英]Java: instantiating an enum using reflection
Suppose you have a text file like: 假设您有一个文本文件,如:
my_setting = ON
some_method = METHOD_A
verbosity = DEBUG
...
That you wish to to update a corresponding object accordingly: 您希望相应地更新相应的对象:
Setting my_setting = ON;
Method some_method = METHOD_A;
Verbosity verbosity = DEBUG;
...
Where all are different kind of enums. 所有都是不同类型的枚举。
I would like to have a generic way to instantiate the enum values. 我想有一个通用的方法来实例化枚举值。 That is, at runtime using reflection, and without knowing the enum types of the object in advance. 也就是说,在运行时使用反射,并且事先不知道对象的枚举类型。
I would have imagined something like this: 我会想象这样的事情:
for (ConfigLine line : lines)
{
String[] tokens = line.string.split("=", 2);
String name = tokens[0].trim();
String value = tokens[1].trim();
try
{
Field field = this.getClass().getDeclaredField(name);
if(field.getType().isEnum())
{
// doesn't work (cannot convert String to enum)
field.set(this, value);
// invalid code (some strange generics issue)
field.set(this, Enum.valueOf(field.getType().getClass(), value));
}
else
{ /*...*/ }
}
catch //...
}
The question is: what should there be instead? 问题是:应该有什么? Is it even possible to instantiate an unknown enum given its String representation? 是否有可能在给定String表示的情况下实例化一个未知的枚举?
field.set(this, Enum.valueOf((Class<Enum>) field.getType(), value));
getClass()
after getType()
should not be called - it returns the class of a Class
instance 不应该调用getType()
getClass()
之后的getClass()
- 它返回Class
实例的Class
Class<Enum>
, to avoid generic problems, because you already know that the Class
is an enum
您可以强制转换Class<Enum>
,以避免泛型问题,因为您已经知道Class
是enum
Alternative solution with no casting 没有铸造的替代解决方案
try {
Method valueOf = field.getType().getMethod("valueOf", String.class);
Object value = valueOf.invoke(null, param);
field.set(test, value);
} catch ( ReflectiveOperationException e) {
// handle error here
}
你有一个额外的getClass
调用,你必须施放(每个Bozho更具体的施法):
field.set(test, Enum.valueOf((Class<Enum>) field.getType(), value));
You may code your Enum similar tho this: 您可以使用以下代码编写您的Enum代码:
public enum Setting {
ON("ON"),OFF("OFF");
private final String setting;
private static final Map<String, Setting> stringToEnum = new ConcurrentHashMap<String, Setting>();
static {
for (Setting set: values()){
stringToEnum.put(set.setting, set);
}
}
private Setting(String setting) {
this.setting = setting;
}
public String toString(){
return this.setting;
}
public static RequestStatus fromString(String setting){
return stringToEnum.get(setting);
}
}
Then you may easily create Enum from String without reflection: 然后你可以轻松地从String创建Enum而不用反射:
Setting my_settings = Setting.fromString("ON");
This solution is not originated from myself. 这个解决方案不是源自我自己。 I read it from somewhere else, but I can't recall the source. 我是从其他地方读到的,但我记不起来了。
The accepted answer results in warnings because it uses the raw type Enum instead of Enum<T extends Enum<T>>. 接受的答案会产生警告,因为它使用原始类型Enum而不是Enum <T extends Enum <T >>。
To get around this you need to use a generic method like this: 要解决这个问题,你需要使用这样的通用方法:
@SuppressWarnings("unchecked")
private <T extends Enum<T>> T createEnumInstance(String name, Type type) {
return Enum.valueOf((Class<T>) type, name);
}
Call it like this: 像这样称呼它:
Enum<?> enum = createEnumInstance(name, field.getType());
field.set(this, enum);
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