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查看JSONObject源码：

 @Override
    public String toString() {
        return toJSONString();
    }

    public String toJSONString() {
        SerializeWriter out = new SerializeWriter();
        try {
            new JSONSerializer(out).write(this);
            return out.toString();
        } finally {
            out.close();
        }
    }

其实toString()方法内部还是调用了toJSONString()方法