java Java如何使用反射检查字段是否已初始化或是否为默认值？
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Java how can I with reflection check if a field is initialized or is default value?
提问by rapadura
So, should be straight forward question.
所以，应该是直截了当的问题。
Lets say I have a class with a lot of fields like:
假设我有一个包含很多字段的类，例如：
String thizz;
long that;
boolean bar;
How can I, with reflection, see if the fields thizz, thatand barhave been initialized or left to their default values of null, 0 and false?
我如何通过反射查看字段thizz,that和bar是否已初始化或保留为它们的默认值 null、0 和 false？
回答by Peter Lawrey
You have only 7 primitive types and one reference type to check. If you group all Number types together, you only have four values to check for.
您只有 7 种基本类型和一种引用类型需要检查。如果将所有 Number 类型组合在一起，则只有四个值需要检查。
Object o =
for (Field field : o.getClass().getDeclaredFields()) {
Class t = field.getType();
Object v = field.get(o);
if(t == boolean.class && Boolean.FALSE.equals(v))
// found default value
else if(t == char.class && ((Character) v).charValue() == 0)
// found default value
else if(t.isPrimitive() && ((Number) v).doubleValue() == 0)
// found default value
else if(v == null)
// found default value
}
回答by Jeffrey
You don't need reflection...
你不需要反思...
if (thizz == null) {
//it's not initialized
}
if (that == 0) {
//it's not initialized
}
if(bar == false) {
//it's not initialized
}
However, they could have been initialized then reset to their default values. If you truly want to know if they're been initialized you could do something like this:
但是，它们可以被初始化然后重置为它们的默认值。如果您真的想知道它们是否已初始化，您可以执行以下操作：
private boolean isFooInitialized = false;
private Foo foo;
public void setFoo(Foo foo) {
this.foo = foo;
isFooInitialized = foo != null;
}
/edit To get all the fields from a class, check out Class.getDeclaredFields(). This will give every field, not just the public ones.
/edit 要从类中获取所有字段，请查看Class.getDeclaredFields()。这将提供每个领域，而不仅仅是公共领域。
From here you can check the type of the field and get its value:
从这里您可以检查字段的类型并获取其值：
Foo foo = ...
Field[] fooFields = foo.getClass().getDeclaredFields();
for (Field fooField : fooFields) {
Class<?> fooFieldClass = fooField.getClass();
if (fooFieldClass.equals(int.class)) {
if (fooField.getInt(foo) == 0) {
// not initialized
}
} else if (fooFieldClass.equals(double.class)) {
if (fooField.getDouble(foo) == 0) {
// not initialized
}
} else if (fooFieldClass.equals(boolean.class)) {
if (fooField.getBoolean(foo) == false) {
// not initialized
}
} else if (fooFieldClass.equals(float.class)) {
if (fooField.getFloat(foo) == 0) {
// not initialized
}
} else if (fooFieldClass.equals(char.class)) {
if (fooField.getChar(foo) == 0) {
// not initialized
}
} else if (fooFieldClass.equals(byte.class)) {
if (fooField.getByte(foo) == 0) {
// not initialized
}
} else if (fooFieldClass.equals(long.class)) {
if (fooField.getLong(foo) == 0) {
// not initialized
}
} else if (fooField.get(foo) == null) {
// not initialized
}
}
回答by Christophe Weis
Peter Lawrey's answerworks fine for me, except for fields of primitive data type char, which raises the following exception in my code :
Peter Lawrey 的回答对我来说很好，除了原始数据类型的字段char，这会在我的代码中引发以下异常：
java.lang.ClassCastException: java.lang.Character cannot be cast to java.lang.Number
So I added the charcase :
所以我添加了char案例：
Object o =
for (Field field : o.getClass().getDeclaredFields()) {
Class t = field.getType();
Object v = field.get(o);
if (boolean.class.equals(t) && Boolean.FALSE.equals(v))
// found default value
else if (char.class.equals(t) && ((Character) v) != Character.MIN_VALUE)
// found default value
else if (t.isPrimitive() && ((Number) v).doubleValue() == 0)
// found default value
else if(!t.isPrimitive() && v == null)
// found default value
}
Character.MIN_VALUEis '\u0000', which is the default value of characcording to the Java Documentation on primitive data types.
Character.MIN_VALUEis '\u0000'，这是char根据关于原始数据类型的Java 文档的默认值。
回答by JB Cha
You can create a new instance via reflection (automatically initialized with default value) and compare yourObject with this default instance.
您可以通过反射创建一个新实例（使用默认值自动初始化）并将 yourObject 与此默认实例进行比较。
//default Object filled with default values:
Object defaultObject = yourObject.getClass().newInstance();
for (final Field field : yourObject.getClass().getDeclaredFields()) {
final Object value = field.get(yourObject);
if (value == null || value.equals(field.get(defaultObject))) {
// found default value
}
}
PS: This solution also solved the issue with char mentioned by Christophe Weis.
PS：这个解决方案也解决了 Christophe Weis 提到的 char 问题。
回答by Preston
This should get you going in the right direction.
这应该让你朝着正确的方向前进。
Class clazz = Class.forName("your.class");
Field[] fields = clazz.getDeclaredFields();
for (Field field : fields) {
String dataType = field.getType().getName();
if (dataType.equals("java.lang.String")) {
System.out.println("found a string");
}
}
回答by david van brink
The gist of it is:
它的要点是：
Field[] fields = yourObject.getClass().getFields();
for(Field f : fields)
{
Class<?> k = f.getType();
// depending on k, methods like f.getInt(yourObject),
// f.getFloat(yourObject),
// f.getObject(hourObject) to get each member.
}
Now, this only lets you read the public fields.
现在，这只允许您读取公共字段。
Alternatively, IF your object follows getX/setX naming conventions, you can use getMethods(), and look for methods named "getXxx" and "setXxx", to infer the existence of settable fields -- and invoke those getters to look for the expected default values.
或者，如果您的对象遵循 getX/setX 命名约定，您可以使用 getMethods()，并查找名为“getXxx”和“setXxx”的方法，以推断可设置字段的存在——并调用这些 getter 来查找预期的默认值。
