didi/DoKit · error · NullPointerException

field == null

Error message

field == null

What it means

setRequestProperty() rejects a null header name with NullPointerException("field == null") per the HttpURLConnection API contract; the check runs after the connected check but before the value null-check (a null value is silently ignored instead). Any header key derived from a nullable source can trip this.

Source

Thrown at Android/dokit/src/main/java/com/didichuxing/doraemonkit/aop/urlconnection/ObsoleteUrlFactory.java:688

        }

        @Override
        public String getResponseMessage() throws IOException {
            return getResponse(true).message();
        }

        @Override
        public int getResponseCode() throws IOException {
            return getResponse(true).code();
        }

        @Override
        public void setRequestProperty(String field, String newValue) {
            if (connected) {
                throw new IllegalStateException("Cannot set request property after connection is made");
            }
            if (field == null) {
                throw new NullPointerException("field == null");
            }
            if (newValue == null) {
                return;
            }

            requestHeaders.set(field, newValue);
        }

        @Override
        public void setIfModifiedSince(long newValue) {
            super.setIfModifiedSince(newValue);
            if (ifModifiedSince != 0) {
                requestHeaders.set("If-Modified-Since", format(new Date(ifModifiedSince)));
            } else {
                requestHeaders.removeAll("If-Modified-Since");
            }
        }

View on GitHub (pinned to 626827cddb)

Solutions

1. Null- or blank-check header names before calling setRequestProperty.
2. Filter null keys when building the header map: headers.keySet().removeIf(Objects::isNull).
3. Fix the upstream producer of the null header name (config or model field).

Example fix

// before
for (Map.Entry<String,String> e : headers.entrySet()) {
  conn.setRequestProperty(e.getKey(), e.getValue()); // key null -> NPE
}

// after
for (Map.Entry<String,String> e : headers.entrySet()) {
  if (e.getKey() != null) conn.setRequestProperty(e.getKey(), e.getValue());
}

if (field != null && !field.trim().isEmpty()) conn.setRequestProperty(field, value);

static boolean isValidHeaderName(String s) { return s != null && !s.isEmpty() && s.indexOf(':') < 0; }

When it happens

Trigger: Calling setRequestProperty(null, value), typically because the header name came from a map entry, config key, or variable that is null.

Common situations: Iterating a Map<String,String> of headers where a key is null; building header names from string concatenation or config lookups that can return null.

Related errors

• Cannot add request property after connection is made
• namesAndValues == null
• Headers cannot be null
• headers == null
• name == null

AI-assisted analysis of didi/DoKit@626827cddb (2026-08-14). Data as JSON: /api/errors/0f9ff01d68271eb9. Report an issue: GitHub.