Debugging NPEs in Spring Boot: Real‑World Case Study and Fixes

Introduction

A new intermediate Java developer was assigned a simple task: pull data from a third‑party source, match it to internal channels, aggregate into a list, and batch insert into the database. After deployment, a NullPointerException (NPE) occurred.

Accident Reproduction

Pseudo Code

说明 ：The pseudo code is not real production code but illustrates the NPE scenario. The real business logic is more complex, and NPEs often hide deep in the code.

String channelNo = channelDao.getOne().getChannelNo();
List<ThirdData> thirdDataList = httpClientUtils.getThirdDatas(DateUtils.today());
thirdDataList.stream().filter(o -> channelNo.equals(o.getChannelNo())).collect(Collectors.toList());
thirdDataDao.saveAll(thirdDataList);

Analysis and Solutions

First Line Analysis

If channelDao.getOne() returns null, calling getChannelNo() triggers an NPE.

Solution 1 – Defensive Programming (recommended)

// Defensive check
Channel channel = channelDao.getOne();
if (channel == null) {
    return;
}

Solution 2 – Ternary Operator

String channelNo = channelDao.getOne() == null ? "" : channelDao.getOne().getChannelNo();

Solution 3 – Optional

String channelNo = Optional.ofNullable(channelDao.getOne()).orElse("");

Third Line Analysis (1)

If thirdDataList is null, invoking stream() causes an NPE.

Solution – Collection Null/Empty Check

// Recommended: use collection utility to check emptiness
if (CollectionUtils.isEmpty(thirdDataList)) {
    return;
}

Third Line Analysis (2)

If channelNo is null, the expression channelNo.equals(o.getChannelNo()) throws an NPE.

Solution 1 – Explicit Null Check

channelNo != null && channelNo.equals(o.getChannelNo())

Solution 2 – Objects.equals

Objects.equals(channelNo, o.getChannelNo())

Solution 3 – Utility Libraries

// Apache Commons Lang
StringUtils.equals(channelNo, o.getChannelNo());
// Hutool
StrUtil.equals(channelNo, o.getChannelNo());

Final Recommendation

Install the SonarLint plugin for IntelliJ IDEA. It dynamically detects code risks such as NPEs and provides real‑time feedback. Its server counterpart is SonarQube.

Author: l拉不拉米 (Source: juejin.cn)