获取和设置字段

使用 Reflection API，可以在运行时更改或获取字段的值。例如，你可以在 API 中使用它来根据操作系统等因素检索不同的字段。你还可以删除像 final 这样的修饰符，以允许修改最终的字段。

为此，你需要使用方法 Class#getField() ，方法如下所示：

// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");

// Get the field in class Field "modifiers". Note that it does not 
// need to be static
Field modifiersField = Field.class.getDeclaredField("modifiers");

// Allow access from anyone even if it's declared private
modifiersField.setAccessible(true);

// Get the modifiers on the "NAME" field as an int.
int existingModifiersOnNameField = nameField.getModifiers();

// Bitwise AND NOT Modifier.FINAL (16) on the existing modifiers
// Readup here https://en.wikipedia.org/wiki/Bitwise_operations_in_C
// if you're unsure what bitwise operations are.
int newModifiersOnNameField = existingModifiersOnNameField & ~Modifier.FINAL;

// Set the value of the modifiers field under an object for non-static fields
modifiersField.setInt(nameField, newModifiersOnNameField);

// Set it to be accessible. This overrides normal Java 
// private/protected/package/etc access control checks.
nameField.setAccessible(true);

 // Set the value of "NAME" here. Note the null argument. 
 // Pass null when modifying static fields, as there is no instance object
nameField.set(null, "Hacked by reflection...");

// Here I can directly access it. If needed, use reflection to get it. (Below)
System.out.println(SomeClass.NAME);

获得田地更容易。我们可以使用 Field#get() 及其变体来获取它的值：

// Get the field in class SomeClass "NAME".
Field nameField = SomeClass.class.getDeclaredField("NAME");

// Set accessible for private fields
nameField.setAccessible(true);

// Pass null as there is no instance, remember?
String name = (String) nameField.get(null);

请注意这个：

使用 Class#getDeclaredField 时 ，使用它来获取类本身中的字段：

class HackMe extends Hacked {
    public String iAmDeclared;
}

class Hacked {
    public String someState;
}

在这里，HackMe#iAmDeclared 被声明为 field。但是，HackMe#someState 不是声明的字段，因为它是从它的超类 Hacked 继承而来的。