问题
起初反射修改String的final变量，按理应该能够修改的，但是发现修改不了？但是如果把String赋值形式改为new String("xxx");，则可以，又是为什么？
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为什么反射修改 final String str = "" 变量修改不了，而 final String str = new String("xxx") 形式可以修改？
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为什么反射修改非static的final变量不消去final不会报错，而static&final变量不消去final修饰符会报错？
结论
- 反射修改final变量和是否static无关
- 反射修改字段值，需要获取Field，底层是FieldAccessor接口，不同类型Field不同FieldAccessor实现，所以会导致非static的final变量不消去final不会报错，而static&final变量不消去final修饰符会报错
- 具体值修改是通过unsafe.putObjectVolatile(var1, this.fieldOffset, var2);，所以修改类型底层数据结构，存储结构有关。因为final String str = "xxx" 是存储在方法区中的常量池中，是常量，而final String str = new String("xxx");也是存储在方法区中，但是是引用类型，所以引用类型的话就可以直接修改引用，而常量则不可以。
- 注意：数组是引用类型。
问题解决方式
问题很奇怪，刚开始想了很久没思路，觉得debug一下
public void set(Object obj, Object value)
throws IllegalArgumentException, IllegalAccessException {
// 检查
if (!override) {
if (!Reflection.quickCheckMemberAccess(clazz, modifiers)){
Class<?> caller = Reflection.getCallerClass();
checkAccess(caller, clazz, obj, modifiers);
}
}
// 获取FieldAccessor，并修改值
getFieldAccessor(obj).set(obj, value);
}
- 修改Field接口：FieldAccessor很关键
public FieldAccessor newFieldAccessor(Field var1, boolean var2) {
checkInitted();
return UnsafeFieldAccessorFactory.newFieldAccessor(var1, var2);
}
static FieldAccessor newFieldAccessor(Field var0, boolean var1) {
Class var2 = var0.getType();
boolean var3 = Modifier.isStatic(var0.getModifiers());
boolean var4 = Modifier.isFinal(var0.getModifiers());
boolean var5 = Modifier.isVolatile(var0.getModifiers());
boolean var6 = var4 || var5;
boolean var7 = var4 && (var3 || !var1);
if(var3) {
UnsafeFieldAccessorImpl.unsafe.ensureClassInitialized(var0.getDeclaringClass());
// return ...;
} else {
// return ...;
}
}
- 在这里可以知道，在获取FieldAccessor的时候已经确定了static和final，不取消final会报错的原因跟FieldAccessor的实现有关
- 这里不考虑var5，所以static final 变量 var7为true，final非static变量var7为false。
if(var3) {
UnsafeFieldAccessorImpl.unsafe.ensureClassInitialized(var0.getDeclaringClass());
return (FieldAccessor)(!var6?
(var2 == Boolean.TYPE? new UnsafeStaticBooleanFieldAccessorImpl(var0):
(var2 == Byte.TYPE?new UnsafeStaticByteFieldAccessorImpl(var0):
(var2 == Short.TYPE?new UnsafeStaticShortFieldAccessorImpl(var0):
(var2 == Character.TYPE?new UnsafeStaticCharacterFieldAccessorImpl(var0):
(var2 == Integer.TYPE?new UnsafeStaticIntegerFieldAccessorImpl(var0):
(var2 == Long.TYPE?new UnsafeStaticLongFieldAccessorImpl(var0):
(var2 == Float.TYPE?new UnsafeStaticFloatFieldAccessorImpl(var0):
(var2 == Double.TYPE?new UnsafeStaticDoubleFieldAccessorImpl(var0):
new UnsafeStaticObjectFieldAccessorImpl(var0))))))))):
(var2 == Boolean.TYPE?new UnsafeQualifiedStaticBooleanFieldAccessorImpl(var0, var7):
(var2 == Byte.TYPE?new UnsafeQualifiedStaticByteFieldAccessorImpl(var0, var7):
(var2 == Short.TYPE?new UnsafeQualifiedStaticShortFieldAccessorImpl(var0, var7):
(var2 == Character.TYPE?new UnsafeQualifiedStaticCharacterFieldAccessorImpl(var0, var7):
(var2 == Integer.TYPE?new UnsafeQualifiedStaticIntegerFieldAccessorImpl(var0, var7):
(var2 == Long.TYPE?new UnsafeQualifiedStaticLongFieldAccessorImpl(var0, var7):
(var2 == Float.TYPE?new UnsafeQualifiedStaticFloatFieldAccessorImpl(var0, var7):
(var2 == Double.TYPE?new UnsafeQualifiedStaticDoubleFieldAccessorImpl(var0, var7):
new UnsafeQualifiedStaticObjectFieldAccessorImpl(var0, var7))))))))));
} else {
return (FieldAccessor)(!var6?(
var2 == Boolean.TYPE?new UnsafeBooleanFieldAccessorImpl(var0):
(var2 == Byte.TYPE?new UnsafeByteFieldAccessorImpl(var0):
(var2 == Short.TYPE?new UnsafeShortFieldAccessorImpl(var0):
(var2 == Character.TYPE?new UnsafeCharacterFieldAccessorImpl(var0):
(var2 == Integer.TYPE?new UnsafeIntegerFieldAccessorImpl(var0):
(var2 == Long.TYPE?new UnsafeLongFieldAccessorImpl(var0):
(var2 == Float.TYPE?new UnsafeFloatFieldAccessorImpl(var0):
(var2 == Double.TYPE?new UnsafeDoubleFieldAccessorImpl(var0):
new UnsafeObjectFieldAccessorImpl(var0))))))))):
(var2 == Boolean.TYPE?new UnsafeQualifiedBooleanFieldAccessorImpl(var0, var7):
(var2 == Byte.TYPE?new UnsafeQualifiedByteFieldAccessorImpl(var0, var7):
(var2 == Short.TYPE?new UnsafeQualifiedShortFieldAccessorImpl(var0, var7):
(var2 == Character.TYPE?new UnsafeQualifiedCharacterFieldAccessorImpl(var0, var7):
(var2 == Integer.TYPE?new UnsafeQualifiedIntegerFieldAccessorImpl(var0, var7):
(var2 == Long.TYPE?new UnsafeQualifiedLongFieldAccessorImpl(var0, var7):
(var2 == Float.TYPE?new UnsafeQualifiedFloatFieldAccessorImpl(var0, var7):
(var2 == Double.TYPE?new UnsafeQualifiedDoubleFieldAccessorImpl(var0, var7):
new UnsafeQualifiedObjectFieldAccessorImpl(var0, var7))))))))));
}
-
对于static、final变量，var3为true，var7为true，会创建UnsafeQualifiedStaticObjectFieldAccessorImpl类
-
对于非static变量，var3为false，var7为true，会创建UnsafeQualifiedObjectFieldAccessorImpl类
static final的FieldAccessor
UnsafeQualifiedStaticObjectFieldAccessorImpl
// UnsafeQualifiedStaticObjectFieldAccessorImpl 父类
abstract class UnsafeQualifiedStaticFieldAccessorImpl extends UnsafeStaticFieldAccessorImpl {
protected final boolean isReadOnly;
UnsafeQualifiedStaticFieldAccessorImpl(Field var1, boolean var2) {
super(var1);
this.isReadOnly = var2;
}
}
- 可以看到这里的 isReadOnly 为true
设置Field值方法
public void set(Object var1, Object var2) throws IllegalArgumentException, IllegalAccessException {
if(this.isReadOnly) {
// 错误所在
this.throwFinalFieldIllegalAccessException(var2);
}
if(var2 != null && !this.field.getType().isAssignableFrom(var2.getClass())) {
this.throwSetIllegalArgumentException(var2);
}
unsafe.putObjectVolatile(this.base, this.fieldOffset, var2);
}
- 可以看到，会先判断 isReadOnly变量（是否只读），如果没有取消final修饰符则会在这里抛出异常
- 具体值的修改是通过 unsafe.putObjectVolatile(var1, this.fieldOffset, var2);进行。
final非static的FieldAccessor
UnsafeQualifiedObjectFieldAccessorImpl
// UnsafeQualifiedObjectFieldAccessorImpl 父类，同UnsafeQualifiedStaticObjectFieldAccessorImpl
abstract class UnsafeQualifiedFieldAccessorImpl extends UnsafeFieldAccessorImpl {
protected final boolean isReadOnly;
UnsafeQualifiedFieldAccessorImpl(Field var1, boolean var2) {
super(var1);
this.isReadOnly = var2;
}
}
- 这里的 isReadOnly 上面讲为false，所以不会判断是否可读
参考：
- Why does reflection fail to update a static field?
如果上述有哪些错误或者不足，或者其他方面的思考，希望您能够指出，Thx！
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