3-8、常见异常汇总

java.lang.NullPointerException

java.lang.NullPointerException: Cannot invoke "String.length()" because "str" is null

// 1. 对null对象调用方法
String str = null;
str.length();  // 报错

// 2. 对null集合操作
List<String> list = null;
list.size();  // 报错

// 3. 从Map取值没判空
Map<String, Object> map = new HashMap<>();
String value = (String) map.get("key");
value.length();  // key不存在，返回null

// 4. 依赖注入失败
@Autowired
private UserService userService;  // userService为null

// 方案1：if判空
if (str != null) {
    str.length();
}

// 方案2：Optional
Optional.ofNullable(str)
    .map(String::length)
    .orElse(0);

// 方案3：StringUtils（Apache Commons Lang）
if (StringUtils.isNotEmpty(str)) {
    str.length();
}

// 方案4：集合判空
if (list != null && !list.isEmpty()) {
    list.size();
}

// 或用CollectionUtils
if (CollectionUtils.isNotEmpty(list)) {
    list.size();
}

java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Integer

// 1. 从Map/Redis取值强转
Object obj = map.get("age");
Integer age = (Integer) obj;  // obj实际是String "25"

// 2. 从JSON解析强转
Object obj = JSON.parse(jsonStr);
List<User> list = (List<User>) obj;  // obj实际是JSONObject

// 方案1：用instanceof判断
if (obj instanceof Integer) {
    Integer age = (Integer) obj;
}

// 方案2：用String.valueOf转成String再解析
String str = String.valueOf(obj);
Integer age = Integer.parseInt(str);

// 方案3：用类型安全的JSON解析
List<User> list = JSON.parseArray(jsonStr, User.class);
User user = JSON.parseObject(jsonStr, User.class);

// 方案4：泛型
Map<String, Object> map = new HashMap<>();
// 取值时用String.valueOf，不要强转
String value = String.valueOf(map.get("key"));

java.lang.NumberFormatException: For input string: "abc"

// 字符串不是数字，但用parseInt转换
String str = "abc";
int num = Integer.parseInt(str);  // 报错

// 从前端传来的参数不是数字
String pageStr = request.getParameter("page");
int page = Integer.parseInt(pageStr);  // 如果pageStr是null或非数字

// 方案1：try-catch
try {
    int num = Integer.parseInt(str);
} catch (NumberFormatException e) {
    num = 0;  // 默认值
}

// 方案2：用StringUtils判断
if (StringUtils.isNumeric(str)) {
    int num = Integer.parseInt(str);
}

// 方案3：用NumberUtils（Apache Commons Lang）
int num = NumberUtils.toInt(str, 0);  // 转不了返回默认值0

java.sql.SQLSyntaxErrorException: Table 'my_project.user' doesn't exist

java.sql.SQLSyntaxErrorException: Unknown column 'user_name' in 'field list'

java.sql.SQLException: Data too long for column 'name' at row 1

ALTER TABLE user MODIFY COLUMN name VARCHAR(255);

java.sql.SQLIntegrityConstraintViolationException: Duplicate entry 'admin' for key 'uk_username'

java.sql.SQLIntegrityConstraintViolationException: Cannot delete or update a parent row:
a foreign key constraint fails

java.lang.IllegalArgumentException: Source must not be null

// 1. BeanUtils.copyProperties源对象为null
BeanUtils.copyProperties(null, target);  // 报错

// 2. 参数校验失败
public void process(String name) {
    Assert.notNull(name, "name不能为空");
}

// 1. 判空后再操作
if (source != null) {
    BeanUtils.copyProperties(source, target);
}

// 2. 用Optional
Optional.ofNullable(source)
    .ifPresent(s -> BeanUtils.copyProperties(s, target));

// 3. 用Spring的@Validated参数校验
@PostMapping("/add")
public Result add(@Validated @RequestBody UserDTO dto) {
    // 如果dto字段不符合规则，自动返回400
}

java.io.FileNotFoundException: class path resource [application.yml] cannot be opened

java.io.IOException: Stream Closed

try (InputStream is = new FileInputStream("file.txt")) {
    // 使用流
} catch (IOException e) {
    log.error("读取文件失败", e);
}
// 流自动关闭，不需要手动close

java.util.ConcurrentModificationException

// 遍历集合时同时修改集合
List<String> list = new ArrayList<>(Arrays.asList("a", "b", "c"));
for (String item : list) {
    if ("b".equals(item)) {
        list.remove(item);  // 报错！
    }
}

// 方案1：用Iterator
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    if ("b".equals(it.next())) {
        it.remove();  // 用Iterator的remove方法
    }
}

// 方案2：用removeIf
list.removeIf("b"::equals);

// 方案3：用CopyOnWriteArrayList（线程安全）
List<String> list = new CopyOnWriteArrayList<>(Arrays.asList("a", "b", "c"));

java.lang.StackOverflowError: null

// 1. 无限递归
public int factorial(int n) {
    return n * factorial(n - 1);  // 没有终止条件
}

// 2. 循环依赖
// A类的构造方法new B，B类的构造方法new A

// 1. 递归加终止条件
public int factorial(int n) {
    if (n <= 1) return 1;  // 终止条件
    return n * factorial(n - 1);
}

// 2. 检查循环依赖，用@Autowired代替手动new

java.lang.OutOfMemoryError: Java heap space

java.lang.OutOfMemoryError: GC overhead limit exceeded

java.lang.OutOfMemoryError: Metaspace

org.springframework.beans.factory.BeanCreationException:
Error creating bean with name 'userService'

org.springframework.beans.factory.BeanCurrentlyInCreationException:
Error creating bean with name 'serviceA': Requested bean is currently in creation

NoSuchBeanDefinitionException: No qualifying bean of type 'com.example.UserService' available

BindException: Failed to bind properties under 'spring.datasource' to ...

org.apache.ibatis.binding.BindingException:
Invalid bound statement (not found): com.example.mapper.UserMapper.selectList

org.apache.ibatis.binding.BindingException:
Parameter 'userId' not found. Available parameters are [arg0, param1]

// 错误
List<User> selectByCondition(String name, Integer age);

// 正确
List<User> selectByCondition(@Param("name") String name, @Param("age") Integer age);

com.fasterxml.jackson.databind.JsonMappingException:
Infinite recursion (StackOverflowError)

// 加 @JsonIgnore 忽略反向引用
@JsonIgnore
private Order order;  // User类里引用Order，Order类里引用User

// 或用 @JsonManagedReference / @JsonBackReference
@JsonManagedReference
private List<User> users;  // 一方

@JsonBackReference
private Department department;  // 多方

Cannot deserialize value of type `java.util.Date` from String "2026-01-01"

@JsonFormat(pattern = "yyyy-MM-dd", timezone = "Asia/Shanghai")
private Date birthday;

UnrecognizedPropertyException: Unrecognized field "xxx"

// 忽略未知属性
@JsonIgnoreProperties(ignoreUnknown = true)
public class UserDTO { ... }

spring:
  jackson:
    deserialization:
      fail-on-unknown-properties: false