反射修改 static final 變數

阿新 • • 發佈：2020-07-08

一、測試結論

static final 修飾的基本型別和String型別不能通過反射修改;

二、測試案例

@Test

public void test01() throws Exception {

  setFinalStatic(Constant.class.getDeclaredField("i1"), 11);

  System.out.println(Constant.i1);

  setFinalStatic(Constant.class.getDeclaredField("i2"), 22);

  System.out.println(Constant.i2);

  setFinalStatic(Constant.class.getDeclaredField("s1"), "change1");
 

  System.out.println(Constant.s1);

  setFinalStatic(Constant.class.getDeclaredField("s2"), "change2");

  System.out.println(Constant.s2);

  System.out.println("----------------");

  setFinalStatic(CC.class.getDeclaredField("i1"), 11);

  System.out.println(CC.i1);

  setFinalStatic(CC.class.getDeclaredField("i2"), 22);
 

  System.out.println(CC.i2);

  setFinalStatic(CC.class.getDeclaredField("i3"), 33);

  System.out.println(CC.i3);

  setFinalStatic(CC.class.getDeclaredField("s1"), "change1");

  System.out.println(CC.s1);

  setFinalStatic(CC.class.getDeclaredField("s2"), "change2");
 

  System.out.println(CC.s2);

  setFinalStatic(CC.class.getDeclaredField("s3"), "change3");

  System.out.println(CC.s3);

}

private void setFinalStatic(Field field, Object newValue) throws Exception {

  field.setAccessible(true);

  Field modifiersField = Field.class.getDeclaredField("modifiers");

  modifiersField.setAccessible(true);

  modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

  field.set(null, newValue);

}

interface Constant {

  int i1 = 1;

  Integer i2 = 1;

  String s1 = "s1";

  String s2 = new String("s2");

}

static class CC {

  private static final int i1 = 1;

  private static final Integer i2 = 1;

  private static Integer i3 = 1;

  private static final String s1 = "s1";

  private static final String s2 = new String("s2");

  private static String s3 = "s3";

}

// 列印結果

1

22

s1

change2

----------------

1

22

33

s1

change2

change3

從列印的日誌可以看到,正如開篇所說,除了 static final 修飾的基本型別和String型別修改失敗,其他的都修改成功了;

但是這裡有一個很有意思的現象,在debug的時候顯示 i1 已經修改成功了,但是在列印的時候卻任然是原來的值;

就是因為這個debug然我疑惑了很久,但是仔細分析後感覺這是一個bug,詳細原因還暫時未知;

三、案例分析

private void setFinalStatic(Field field, Object newValue) throws Exception {

  field.setAccessible(true);

  Field modifiersField = Field.class.getDeclaredField("modifiers");

  modifiersField.setAccessible(true);

  modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

  field.set(null, newValue);

}

首先這裡修改 static final 值得原理是,將這個 Field 的 FieldAccessor 的 final 給去掉了,否則在 field.set(null, newValue); 的時候, 就會檢查 final 而導致失敗

// UnsafeIntegerFieldAccessorImpl

if (this.isFinal) {

    this.throwFinalFieldIllegalAccessException(var2);

}

而我們在 CC.class.getDeclaredField("i1") 獲取的 Field 其實是 clazz 物件中的一個備份,

// Class

private static Field searchFields(Field[] fields, String name) {

  String internedName = name.intern();

  for (int i = 0; i < fields.length; i++) {

    if (fields[i].getName() == internedName) {

      return getReflectionFactory().copyField(fields[i]);

    }

  }

  return null;

}

Field copy() {

  if (this.root != null)

    throw new IllegalArgumentException("Can not copy a non-root Field");

  Field res = new Field(clazz, name, type, modifiers, slot, signature, annotations);

  res.root = this;

  // Might as well eagerly propagate this if already present

  res.fieldAccessor = fieldAccessor;

  res.overrideFieldAccessor = overrideFieldAccessor;

  return res;

}

所以在 field.set(null, newValue); 設定新值得時候,這裡就應該是類似值傳遞和引用傳遞的問題,複製出來的 field 其實已經修改成功了,但是 root 物件仍然是原來的值,而在列印的時候,其實是直接取的 root 物件的值;

private void setFinalStatic(Field field, Object newValue) throws Exception {

  field.setAccessible(true);

  // Object o1 = field.get(null);

  Field modifiersField = Field.class.getDeclaredField("modifiers");

  modifiersField.setAccessible(true);

  modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

  field.set(null, newValue);

  Object o1 = field.get(null);

}

// 列印 11

注意如果這裡在去掉 final 之前就取了一次值,就會 set 失敗, 因為 Class 預設開啟了 useCaches 快取, get 的時候會獲取到 root field 的 FieldAccessor, 後面的重設就會失效;

四、位元組碼分析

這個問題還可以從位元組碼的角度分析:

public class CC {

    public static final int i1 = 1;

    public static final Integer i2 = 1;

    public static int i3 = 1;

    public final int i4 = 1;

    public int i5 = 1;

}

// javap -verbose class

警告: 二進位制檔案CC包含com.sanzao.CC

Classfile /Users/wangzichao/workspace/test/target/classes/com/sanzao/CC.class

  Last modified 2020-7-8; size 572 bytes

  MD5 checksum 5f5847cb849315f98177420057130de6

  Compiled from "CC.java"

public class com.sanzao.CC

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = Methodref          #8.#28         // java/lang/Object."<init>":()V

   #2 = Fieldref           #7.#29         // com/sanzao/CC.i4:I

   #3 = Fieldref           #7.#30         // com/sanzao/CC.i5:I

   #4 = Methodref          #31.#32        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

   #5 = Fieldref           #7.#33         // com/sanzao/CC.i2:Ljava/lang/Integer;

   #6 = Fieldref           #7.#34         // com/sanzao/CC.i3:I

   #7 = Class              #35            // com/sanzao/CC

   #8 = Class              #36            // java/lang/Object

   #9 = Utf8               i1

  #10 = Utf8               I

  #11 = Utf8               ConstantValue

  #12 = Integer            1

  #13 = Utf8               i2

  #14 = Utf8               Ljava/lang/Integer;

  #15 = Utf8               i3

  #16 = Utf8               i4

  #17 = Utf8               i5

  #18 = Utf8               <init>

  #19 = Utf8               ()V

  #20 = Utf8               Code

  #21 = Utf8               LineNumberTable

  #22 = Utf8               LocalVariableTable

  #23 = Utf8               this

  #24 = Utf8               Lcom/sanzao/CC;

  #25 = Utf8               <clinit>

  #26 = Utf8               SourceFile

  #27 = Utf8               CC.java

  #28 = NameAndType        #18:#19        // "<init>":()V

  #29 = NameAndType        #16:#10        // i4:I

  #30 = NameAndType        #17:#10        // i5:I

  #31 = Class              #37            // java/lang/Integer

  #32 = NameAndType        #38:#39        // valueOf:(I)Ljava/lang/Integer;

  #33 = NameAndType        #13:#14        // i2:Ljava/lang/Integer;

  #34 = NameAndType        #15:#10        // i3:I

  #35 = Utf8               com/sanzao/CC

  #36 = Utf8               java/lang/Object

  #37 = Utf8               java/lang/Integer

  #38 = Utf8               valueOf

  #39 = Utf8               (I)Ljava/lang/Integer;

{

  public static final int i1;

    descriptor: I

    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL

    ConstantValue: int 1

  public static final java.lang.Integer i2;

    descriptor: Ljava/lang/Integer;

    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL

  public static int i3;

    descriptor: I

    flags: ACC_PUBLIC, ACC_STATIC

  public final int i4;

    descriptor: I

    flags: ACC_PUBLIC, ACC_FINAL

    ConstantValue: int 1

  public int i5;

    descriptor: I

    flags: ACC_PUBLIC

  public com.sanzao.CC();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=2, locals=1, args_size=1

         0: aload_0

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

         4: aload_0

         5: iconst_1

         6: putfield      #2                  // Field i4:I

         9: aload_0

        10: iconst_1

        11: putfield      #3                  // Field i5:I

        14: return

      LineNumberTable:

        line 3: 0

        line 7: 4

        line 8: 9

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      15     0  this   Lcom/sanzao/CC;

  static {};

    descriptor: ()V

    flags: ACC_STATIC

    Code:

      stack=1, locals=0, args_size=0

         0: iconst_1

         1: invokestatic  #4                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;

         4: putstatic     #5                  // Field i2:Ljava/lang/Integer;

         7: iconst_1

         8: putstatic     #6                  // Field i3:I

        11: return

      LineNumberTable:

        line 5: 0

        line 6: 7

}

SourceFile: "CC.java"

   #9 = Utf8               i1

  #10 = Utf8               I

  #11 = Utf8               ConstantValue

  #12 = Integer            1

從這裡就能看到 i1 其實是在編譯的時候就已經初始化了(程式碼內聯)優化, 而 i4, i5 是在建構函式的時候初始化, i2, i3 是在執行 static 階段初始化, 同時 i2, i3, i4, i5 都會指向一個 Fieldref 物件, 所以在執行階段就能通過 Fieldref 反射到它真實的值;

反射修改 static final 變數

一、測試結論

二、測試案例

三、案例分析

四、位元組碼分析

反射修改 static final 變數

JavaSE static final及abstract修飾符例項解析

025：為什麼需要將Logger物件宣告為private static final型別的

【問題集】Static 靜態變數不能直接使用 @autowired標籤的問題

類內static成員變數宣告與定義

[Java基礎]static,final修飾符

Java如何修改.class檔案變數

String例項化及static final修飾符實現方法解析

java使用static靜態變數

PHP面向物件之方法修飾符 (static final abstract)

js 讀取/修改/刪除 css3變數

Java 反射修改類的常量值、靜態變數值、屬性值例項詳解

反射修改String的內容

反射方式獲取成員變數並使用

反射修改字串內容

Java語言-使用static final宣告類常量。

static final關鍵字

java反射修改類註解屬性

迅為-i.MX6開發板手冊更新-非裝置樹uboot-修改預設環境變數

Java中的final變數、final方法和final類

反射修改 static final 變數

一、測試結論

二、測試案例

三、案例分析

四、位元組碼分析

相關推薦