blocks|key|980333|text|final字段的全部意义在于，一旦设置，它就不能重新分配。JVM使用这个guarentee来维护不同地方(例如引用外部变量的内部类)的一致性。所以不是。这样做会破坏JVM！|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|980334|解决方案不是一开始就将其声明为final。|980335|entityMap^0|0|5|0|F|5|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

The whole point of a <code>final</code> field is that it cannot be reassigned once set. The JVM uses this guarentee to maintain consistency in various places (eg inner classes referencing outer variables). So no. Being able to do so would break the JVM!

The solution is not to declare it <code>final</code> in the first place.

blocks|key|980384|text|我还将它与joor+library集成在一起|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|980385|只需使用|980386|++++++Reflect.on(yourObject).set("finalFieldName",+finalFieldValue);|code-block|syntax|javascript|980387|此外，我还修复了override的一个问题，这似乎是以前的解决方案所遗漏的。然而，只有在没有其他好的解决方案的情况下，才要非常小心地使用它。|style|CODE|980388|entityMap|0|LINK|mutability|MUTABLE|url|https://github.com/jOOQ/jOOR^0|5|C|0|0|0|0|8|8|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]]|C|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|12|8|@$A|13|B|14|M|N]]|9|@]|C|$]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|C|$]]]|P|$Q|$5|R|S|T|C|$U|V]]]]

I also integrated it with <a href="https://github.com/jOOQ/jOOR" rel="nofollow noreferrer">joor library</a>

Just use 

<pre><code> Reflect.on(yourObject).set("finalFieldName", finalFieldValue);
</code></pre>

Also I fixed an issue with <code>override</code> which the previous solutions seem to miss.
However use this very carefully, only when there's no other good solution.

blocks|key|980431|text|我刚刚看到了一个面试问题，如果可能的话，用反射或在运行时改变最后一个变量。我真的很感兴趣，所以我开始：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|980432|+/**
+*+@author+Dmitrijs+Lobanovskis
+*+@since+03/03/2016.
+*/
public+class+SomeClass+{

++++private+final+String+str;

++++SomeClass(){
++++++++this.str+=+"This+is+the+string+that+never+changes!";
++++}

++++public+String+getStr()+{
++++++++return+str;
++++}

++++@Override
++++public+String+toString()+{
++++++++return+"Class+name:+"+%2B+getClass()+%2B+"+Value:+"+%2B+getStr();
++++}
}|code-block|syntax|javascript|980433|一些带有最终字符串变量的简单类。所以在主类中导入java.lang.reflect.Field；|980434|/**
+*+@author+Dmitrijs+Lobanovskis
+*+@since+03/03/2016.
+*/
public+class+Main+{


++++public+static+void+main(String[]+args)+throws+Exception{

++++++++SomeClass+someClass+=+new+SomeClass();
++++++++System.out.println(someClass);

++++++++Field+field+=+someClass.getClass().getDeclaredField("str");
++++++++field.setAccessible(true);

++++++++field.set(someClass,+"There+you+are");

++++++++System.out.println(someClass);
++++}
}|980435|输出将如下所示：|980436|Class+name:+class+SomeClass+Value:+This+is+the+string+that+never+changes!
Class+name:+class+SomeClass+Value:+There+you+are

Process+finished+with+exit+code+0|980437|根据文档https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html|offset|length|980438|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html^0|0|0|0|0|0|0|4|1Z|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|12|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|14|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|16|8|@]|9|@$Q|17|R|18|1|19]]|A|$]]|$1|S|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

Just saw that question on one of the interview question, if possible to change final variable with reflection or in runtime. 
Got really interested, so that what I became with:

<pre><code> /**
 * @author Dmitrijs Lobanovskis
 * @since 03/03/2016.
 */
public class SomeClass {

 private final String str;

 SomeClass(){
 this.str = "This is the string that never changes!";
 }

 public String getStr() {
 return str;
 }

 @Override
 public String toString() {
 return "Class name: " + getClass() + " Value: " + getStr();
 }
}
</code></pre>

Some simple class with final String variable. So in the main class 
import java.lang.reflect.Field;

<pre><code>/**
 * @author Dmitrijs Lobanovskis
 * @since 03/03/2016.
 */
public class Main {


 public static void main(String[] args) throws Exception{

 SomeClass someClass = new SomeClass();
 System.out.println(someClass);

 Field field = someClass.getClass().getDeclaredField("str");
 field.setAccessible(true);

 field.set(someClass, "There you are");

 System.out.println(someClass);
 }
}
</code></pre>

The output will be as follows:

<pre><code>Class name: class SomeClass Value: This is the string that never changes!
Class name: class SomeClass Value: There you are

Process finished with exit code 0
</code></pre>

According to documentation 
<a href="https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html" rel="nofollow">https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html</a>

blocks|key|1194792|text|在存在安全管理器的情况下，可以使用AccessController.doPrivileged|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1194793|从上面接受的答案中取相同的例子：|1194794|import+java.lang.reflect.*;

public+class+EverythingIsTrue+{
++++static+void+setFinalStatic(Field+field,+Object+newValue)+throws+Exception+{
++++++++field.setAccessible(true);
++++++++Field+modifiersField+=+Field.class.getDeclaredField("modifiers");

++++++++//+wrapping+setAccessible+
++++++++AccessController.doPrivileged(new+PrivilegedAction()+{
++++++++++++@Override
++++++++++++public+Object+run()+{
++++++++++++++++modifiersField.setAccessible(true);
++++++++++++++++return+null;
++++++++++++}
++++++++});

++++++++modifiersField.setInt(field,+field.getModifiers()+&+~Modifier.FINAL);
++++++++field.set(null,+newValue);
++++}

++++public+static+void+main(String+args[])+throws+Exception+{++++++
++++++setFinalStatic(Boolean.class.getField("FALSE"),+true);
++++++System.out.format("Everything+is+%25s",+false);+//+"Everything+is+true"
++++}
}|code-block|syntax|javascript|1194795|在λ表达式中，AccessController.doPrivileged可以简化为：|1194796|AccessController.doPrivileged((PrivilegedAction)+()+->+{
++++modifiersField.setAccessible(true);
++++return+null;
});|1194797|entityMap^0|H|T|0|0|0|7|T|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|10|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|11|8|@]|D|@]|E|$]]]|R|$]]

In case of presence of a Security Manager, one can make use of <code>AccessController.doPrivileged</code> 

Taking the same example from accepted answer above:

<pre><code>import java.lang.reflect.*;

public class EverythingIsTrue {
 static void setFinalStatic(Field field, Object newValue) throws Exception {
 field.setAccessible(true);
 Field modifiersField = Field.class.getDeclaredField("modifiers");

 // wrapping setAccessible 
 AccessController.doPrivileged(new PrivilegedAction() {
 @Override
 public Object run() {
 modifiersField.setAccessible(true);
 return null;
 }
 });

 modifiersField.setInt(field, field.getModifiers() &amp; ~Modifier.FINAL);
 field.set(null, newValue);
 }

 public static void main(String args[]) throws Exception { 
 setFinalStatic(Boolean.class.getField("FALSE"), true);
 System.out.format("Everything is %s", false); // "Everything is true"
 }
}
</code></pre>

In lambda expression, <code>AccessController.doPrivileged</code>, can be simplified to:

<pre><code>AccessController.doPrivileged((PrivilegedAction) () -&gt; {
 modifiersField.setAccessible(true);
 return null;
});
</code></pre>

blocks|key|980557|text|在JDK+1.8u91上部署之前，这个被接受的答案一直适用于我。然后，当我在调用setFinalStatic方法之前通过反射读取值时，我意识到它在field.set(null,+newValue);行失败。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|980558|可能这次读取导致了不同的Java反射内部设置(即失败情况下的sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl而不是成功情况下的sun.reflect.UnsafeStaticObjectFieldAccessorImpl+)，但我没有详细说明。|980559|因为我需要根据旧值临时设置新值，然后再重新设置旧值，所以我稍微修改了签名，在外部提供了计算函数，同时也返回了旧值：|980560|public+static+<T>+T+assignFinalField(Object+object,+Class<?>+clazz,+String+fieldName,+UnaryOperator<T>+newValueFunction)+{
++++Field+f+=+null,+ff+=+null;
++++try+{
++++++++f+=+clazz.getDeclaredField(fieldName);
++++++++final+int+oldM+=+f.getModifiers();
++++++++final+int+newM+=+oldM+&+~Modifier.FINAL;
++++++++ff+=+Field.class.getDeclaredField("modifiers");
++++++++ff.setAccessible(true);
++++++++ff.setInt(f,newM);
++++++++f.setAccessible(true);

++++++++T+result+=+(T)f.get(object);
++++++++T+newValue+=+newValueFunction.apply(result);

++++++++f.set(object,newValue);
++++++++ff.setInt(f,oldM);

++++++++return+result;
++++}+...|code-block|syntax|javascript|980561|然而，对于一般情况，这是不够的。|980562|entityMap^0|14|E|21|Q|0|U|1K|2N|1B|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@$9|Y|A|Z|B|C]|$9|10|A|11|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|12|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|13|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

The accepted answer worked for me until deployed on JDK 1.8u91.
Then I realized it failed at <code>field.set(null, newValue);</code> line when I had read the value via reflection before calling of <code>setFinalStatic</code> method.

Probably the read caused somehow different setup of Java reflection internals (namely <code>sun.reflect.UnsafeQualifiedStaticObjectFieldAccessorImpl</code> in failing case instead of <code>sun.reflect.UnsafeStaticObjectFieldAccessorImpl</code> in success case) but I didn't elaborate it further.

Since I needed to temporarily set new value based on old value and later set old value back, I changed signature little bit to provide computation function externally and also return old value:

<pre><code>public static &lt;T&gt; T assignFinalField(Object object, Class&lt;?&gt; clazz, String fieldName, UnaryOperator&lt;T&gt; newValueFunction) {
 Field f = null, ff = null;
 try {
 f = clazz.getDeclaredField(fieldName);
 final int oldM = f.getModifiers();
 final int newM = oldM &amp; ~Modifier.FINAL;
 ff = Field.class.getDeclaredField("modifiers");
 ff.setAccessible(true);
 ff.setInt(f,newM);
 f.setAccessible(true);

 T result = (T)f.get(object);
 T newValue = newValueFunction.apply(result);

 f.set(object,newValue);
 ff.setInt(f,oldM);

 return result;
 } ...
</code></pre>

However for general case this would not be sufficient.

blocks|key|1194903|text|除了排名靠前的答案之外，你还可以使用一些最简单的方法。Apache+commons的FieldUtils类已经有了可以做这些事情的特定方法。请看一下FieldUtils.removeFinalModifier方法。您应该指定目标字段实例和辅助功能强制标志(如果您使用非公共字段)。更多信息你可以在here上找到。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1194904|entityMap|0|LINK|mutability|MUTABLE|url|https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/reflect/FieldUtils.html^0|16|A|22|U|44|4|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

Along with top ranked answer you may use a bit simpliest approach. Apache commons <code>FieldUtils</code> class already has particular method that can do the stuff. Please, take a look at <code>FieldUtils.removeFinalModifier</code> method. You should specify target field instance and accessibility forcing flag (if you play with non-public fields). More info you can find <a href="https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/reflect/FieldUtils.html" rel="noreferrer">here</a>.

blocks|key|980621|text|如果你的字段仅仅是私有的，你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|980622|MyClass+myClass=+new+MyClass();
Field+aField=+myClass.getClass().getDeclaredField("someField");
aField.setAccessible(true);
aField.set(myClass,+"newValueForAString");|code-block|syntax|javascript|980623|并抛出/处理NoSuchFieldException|980624|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

If your field is simply private you can do this:

<pre><code>MyClass myClass= new MyClass();
Field aField= myClass.getClass().getDeclaredField("someField");
aField.setAccessible(true);
aField.set(myClass, "newValueForAString");
</code></pre>

and throw/handle NoSuchFieldException

I have a class with a <code>private static final</code> field that, unfortunately, I need to change it at run-time.

Using reflection I get this error: <code>java.lang.IllegalAccessException: Can not set static final boolean field</code>

Is there any way to change the value?

<pre><code>Field hack = WarpTransform2D.class.getDeclaredField("USE_HACK");
hack.setAccessible(true);
hack.set(null, true);
</code></pre>

Change private static final field using Java reflection

Java

我有一个带有private static final字段的类，不幸的是，我需要在运行时更改它。使用反射时，我得到这个错误：java.lang.IllegalAccessException: Can not set static final boolean field有什么方法可以改变这个值吗？Field hack = ...

问使用Java反射更改私有静态最终字段
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