Java Problems Summary

Maven Related Problems

Solving Maven Remote Repository Jar Download Failures

Go to Maven Repository to find the corresponding dependency and download it.

Go to the download directory and execute the following command, replacing -DgroupId, -DartifactId, -Dversion, -Dfile

mvn install:install-file -DgroupId=io.confluent -DartifactId=kafka-schema-registry-client -Dversion=4.1.0 -Dpackaging=jar  -Dfile=kafka-schema-registry-client-4.1.0.jar

mvnw clean occurs Some Enforcer rules have failed

Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:1.4.1:enforce
(enforce-versions) on project SIMI: Some Enforcer rules have failed. Look above for
specific messages explaining why the rule failed. -> [Help 1]

not solved! stackoverflow website

To skip enforcer (not always working)

mvn clean install -Denforcer.skip=true

To continue the build if error

mvn clean install -Denforcer.fail=false

./mvnw clean install -DskipDistribution=true  -Denforcer.skip=true

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MyBatis Related Problems

UncategorizedSQLException when SQL query null value is mapped to int type

Change BO property from int to Integer type

org.springframework.jdbc.UncategorizedSQLException: SqlMapClient operation; 
uncategorized SQLException for SQL []; SQL state [null]; error code [0];

Mysql tinyint is toxic

mysql do not use field type tinyint, possibly your update of 1 will become 49

See blog

MybatisPlus Invalid bound statement (not found) exception

See article

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IDEA Usage Related Problems

idea terminal Chinese garbled characters

Reference article

# solve idea terminal Chinese garbled
export LANG="zh_CN.UTF-8"
export LC_ALL="zh_CN.UTF-8"

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Other Problems

Dynamic Data Source Problems

If the operation used has a transaction, you need to set the data source before the Dao layer. In Dao or Mybatis-plus, service with dao will switch data source invalidly.

jackson serialization problems

LocalDateTime not supported by default

Java 8 date/time type java.time.LocalDateTime not supported by default:
add Module "com.fasterxml.jackson.datatype:jackson-datatype-jsr310" to enable
handling (through reference chain: java.util.ArrayList[0]->xxx.xxx.xxx.xxx.XxxxPo["updatetime"])

First troubleshoot where it is reported

1. If Spring Boot can be extended: directly extend ObjectMapper

@Configuration
public class JacksonConfig implements WebMvcConfigurer {
    @Override
    public void extendMessageConverters(final List<HttpMessageConverter<?>> converters) {
        MappingJackson2HttpMessageConverter messageConverter = new MappingJackson2HttpMessageConverter();
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
        objectMapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
        objectMapper.registerModule(new JavaTimeModule());
        messageConverter.setObjectMapper(objectMapper);
        converters.add(0, messageConverter);
    }
}

2. If cannot extend, consider skipping Bean serialization step, such as using method to convert Bean to Map
3. If really no way, use Fastjson or Gjson instead

Second field uppercase problem

1. Use @JsonProperty

@JsonProperty is used to explicitly tell Jackson:

"This field (or method, constructor parameter) is to be used as a JSON property, and the property name is XXX."

2. Use @JsonAutoDetect

@JsonAutoDetect(
fieldVisibility = JsonAutoDetect.Visibility.ANY,
getterVisibility = JsonAutoDetect.Visibility.NONE,
setterVisibility = JsonAutoDetect.Visibility.NONE)

Members used in serialization and deserialization

Usage Location	Influence Direction	Common Usage
Field	Serialization + Deserialization	Force include private fields
getter	Serialization	Change output field name
setter	Deserialization	Change input field name
Constructor Parameter	Deserialization	Map JSON parameter name to constructor parameter
record Parameter	Serialization + Deserialization	Control JSON property name

Fastjson Date and LocalDateTime timezone issues

Date serializes using the JVM default timezone; LocalDateTime has no timezone info, so its behavior depends on the Fastjson version and JVM settings. Cross-environment timezone mismatch is the main risk.

1. Field-level annotation

Declare format and timezone on DTO/VO fields via @JSONField:

public class UserDTO {
    @JSONField(format = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    private LocalDateTime createTime;

    // Date works the same way
    @JSONField(format = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    private Date createTime;
}

Fastjson's LocalDateTime support varies across versions. In production, prefer converting to Date or String at the API boundary.

2. Spring Boot global config

application.yml (Recommended)

spring:
  jackson:
    date-format: yyyy-MM-dd HH:mm:ss
    time-zone: Asia/Shanghai

Fastjson global config

// Option 1: global defaults
JSON.defaultTimeZone = TimeZone.getTimeZone("Asia/Shanghai");
JSON.defaultDateFormat = "yyyy-MM-dd HH:mm:ss";

// Option 2: FastJsonConfig Bean
@Bean
public FastJsonConfig fastJsonConfig() {
    FastJsonConfig config = new FastJsonConfig();
    config.setDateFormat("yyyy-MM-dd HH:mm:ss");
    config.setCharset(StandardCharsets.UTF_8);
    return config;
}

3. Behavior without explicit timezone

Date d = new Date(0L);
JSON.toJSONString(d);
// UTC env → "1970-01-01 00:00:00"
// Asia/Shanghai env → "1970-01-01 08:00:00"

LocalDateTime has no timezone info, so serialization omits +08:00 or Z:

LocalDateTime ldt = LocalDateTime.of(2024, 1, 1, 12, 0, 0);
JSON.toJSONString(ldt); // "2024-01-01 12:00:00"
// Must specify ZoneId explicitly when converting to Instant
Instant instant = ldt.atZone(ZoneId.of("Asia/Shanghai")).toInstant();

4. Type selection guide

Type	Timezone	Use case
Date	Yes (UTC timestamp)	Cross-timezone transfer, JSON serialization
Instant	Yes (UTC)	Service-internal time calculation, logging
ZonedDateTime	Yes	Business logic that needs timezone
OffsetDateTime	Yes	API responses, cross-system transfer
LocalDateTime	No	Display only, not for transfer or calculation

Prefer Instant / ZonedDateTime / OffsetDateTime inside services. Avoid passing raw Date and LocalDateTime directly as JSON.

Database Connection Pool Configuration Problems

When using Druid connection pool, config parameter validationInterval must be less than timeBetweenEvictionRunMillis, ensuring database connection is always available.

API Interface Standards

• Query data volume
• Input parameter check
• Response message large integer, floating point fields use string type
• Input parameter type not primitive type

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