得瑟一下jdk源代码解析，如何在运行时修改java final变量

先让各位看官看看sun的jdk Field类的代码，

/*
 * Copyright (c) 1996, 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.lang.reflect;

import sun.reflect.FieldAccessor;
import sun.reflect.Reflection;
import sun.reflect.generics.repository.FieldRepository;
import sun.reflect.generics.factory.CoreReflectionFactory;
import sun.reflect.generics.factory.GenericsFactory;
import sun.reflect.generics.scope.ClassScope;
import java.lang.annotation.Annotation;
import java.util.Map;
import sun.reflect.annotation.AnnotationParser;


/**
 * A {@code Field} provides information about, and dynamic access to, a
 * single field of a class or an interface.  The reflected field may
 * be a class (static) field or an instance field.
 *
 * <p>A {@code Field} permits widening conversions to occur during a get or
 * set access operation, but throws an {@code IllegalArgumentException} if a
 * narrowing conversion would occur.
 *
 * @see Member
 * @see java.lang.Class
 * @see java.lang.Class#getFields()
 * @see java.lang.Class#getField(String)
 * @see java.lang.Class#getDeclaredFields()
 * @see java.lang.Class#getDeclaredField(String)
 *
 * @author Kenneth Russell
 * @author Nakul Saraiya
 */
public final
class Field extends AccessibleObject implements Member {

    private Class<?>            clazz;
    private int                 slot;
    // This is guaranteed to be interned by the VM in the 1.4
    // reflection implementation
    private String              name;
    private Class<?>            type;
    private int                 modifiers;
    // Generics and annotations support
    private transient String    signature;
    // generic info repository; lazily initialized
    private transient FieldRepository genericInfo;
    private byte[]              annotations;
    // Cached field accessor created without override
    private FieldAccessor fieldAccessor;
    // Cached field accessor created with override
    private FieldAccessor overrideFieldAccessor;
    // For sharing of FieldAccessors. This branching structure is
    // currently only two levels deep (i.e., one root Field and
    // potentially many Field objects pointing to it.)
    private Field               root;

  .....以下省略

下面的这个函数可以从类的同名文件中载入一个Properties配置，将类的string类型变量赋予一个配置值，即使这个字段是final类型的也是可以的，看了上面代码的看官也许没反应过来，且听分说：

1.首先是Field类的modifiers字段设置为可访问（这个字段本来是private 是不可访问的），获得想要修改字段的Filed对象， 利用开放权限的Filed.modifiers的Filed利用反射修改字段的修饰符去掉final关键字，然后修改变量值，接着就是打扫战场，恢复原来的权限和final修饰

接着说下局限性，只在sun的jre 1.6 1.7 上测试过，其他的jre不敢保证，这个方法不推荐使用，但是你的代码担心被反编译，就把关键变量设置成final然后在需要的时候动态修改。记得把动态修改的代码用自定义classloader 加载进来，用asm动态生成代码也可以，这肯定让分析人看的直晕~~~

protected void load_conf() {
// copy all right , [e-mail], 2011
String file = this.getClass().getName() + ".conf";
//file = this.getClass().getClassLoader().getResource(file).getFile();
final Properties conf = new Properties();
try {
conf.load(new FileInputStream(file));
} catch (final Exception e) {
e.printStackTrace();
return;
}
for (final Field field : this.getClass().getDeclaredFields()) {
if ((field.getType() == String.class)) {
final String fname = field.getName();
final String fvalue = conf.getProperty(fname);
if ((null == fvalue) || (0 == fvalue.length())) {
continue;
}
try {
field.setAccessible(true);
final Field modifiersField = Field.class.getDeclaredField("modifiers");
modifiersField.setAccessible(true);
modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
field.set(this, fvalue);
modifiersField.setInt(field, field.getModifiers() | Modifier.FINAL);
modifiersField.setAccessible(false);
field.setAccessible(false);
} catch (final Exception e) {
e.printStackTrace();
}
}
}
}