Field的public void setAccessible(boolean flag) 说明

   * Set the {@code accessible} flag for this object to
* the indicated boolean value.  A value of {@code true} indicates that
* the reflected object should suppress Java language access
* checking when it is used.  A value of {@code false} indicates
* that the reflected object should enforce Java language access checks.

如果flag设置为true，那么就会压制java语言的访问校验。

Java String 类定义

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public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /** The value is used for character storage. */
    private final char value[];
}

测试代码:

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private static void directChangeStringFinalRef() throws Exception {
        String a = "";
        Field filed = a.getClass().getDeclaredField("value");
        filed.setAccessible(true); //false
        filed.set(a,new char[]{'1','2'});
        System.out.println(a);
    }

设置为false时，机会出现如下异常：

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Exception in thread "main" java.lang.IllegalAccessException: Class book.base.collections.HelloCollections can not access a member of class java.lang.String with modifiers "private final"
	at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:102)
	at java.lang.reflect.AccessibleObject.slowCheckMemberAccess(AccessibleObject.java:296)
	at java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:288)
	at java.lang.reflect.Field.set(Field.java:761)
	at book.base.collections.HelloCollections.directChangeStringFinalRef(HelloCollections.java:93)