獲取和設定欄位

使用 Reflection API，可以在執行時更改或獲取欄位的值。例如，你可以在 API 中使用它來根據作業系統等因素檢索不同的欄位。你還可以刪除像 final 這樣的修飾符，以允許修改最終的欄位。

為此，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);

獲得田地更容易。我們可以使用 Field#get() 及其變體來獲取它的值：

// 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);

請注意這個：

使用 Class#getDeclaredField 時 ，使用它來獲取類本身中的欄位：

class HackMe extends Hacked {
    public String iAmDeclared;
}

class Hacked {
    public String someState;
}

在這裡，HackMe#iAmDeclared 被宣告為 field。但是，HackMe#someState 不是宣告的欄位，因為它是從它的超類 Hacked 繼承而來的。