No modifications are allowed to a locked ParameterMap

错误：java.lang.IllegalStateException: No modifications are allowed to a locked ParameterMap

at org.apache.catalina.util.ParameterMap.remove(ParameterMap.java:205)

1.Servlet中使用了这个方法：request.getParameterMap()，将它的返回值赋值给一个空的hashmap对象结果报错。参考：

it looks like you are trying to *modify* the > received form parameters Map, which under Tomcat's interpretation of > HTTP request handling is a no-no. (The Servlet Spec 3.0 does not > explicitly say so (Chapter 3), but strongly hints in that direction > (there are only "get" methods, no "set" ones).

貌似Servlet规范隐晦的告诉了我们这点。

The relevant info is in the Javadoc for ServletRequest#getParameterMap(). It states quite clearly that the parameter map is immutable.

这个方法的返回值是不可变的。

作者还说一些关键信息却只能在javadoc里找到真是令人抓狂啊～

2.解决方法是：new HashMap(request.getParameterMap())

因为直接用等好赋值只是地址引用，并未改变内存空间，因此无法对之前产生的那个被锁定的map对象进行操作，而使用new方法确实重新申请了内存地址，将该“不可变”的map对象复制到另一个域空间里。

参考：

1.​​http://goo.gl/pvweu​​

2.​​http://goo.gl/AEvPN​​

不能被修改的原因：

org.apache.catalina.util.ParameterMap

/**
     * Associate the specified value with the specified key in this map.  If
     * the map previously contained a mapping for this key, the old value is
     * replaced.
     *
     * @param key Key with which the specified value is to be associated
     * @param value Value to be associated with the specified key
     *
     * @return The previous value associated with the specified key, or
     *  <code>null</code> if there was no mapping for key
     *
     * @exception IllegalStateException if this map is currently locked
     */
    @Override
    public V put(K key, V value) {

        if (locked)
            throw new IllegalStateException
                (sm.getString("parameterMap.locked"));
        return (super.put(key, value));

    }