MySQL added fields but Java entity not updated has potential problems and solutions

MySQL fields are added but Java entities are not updated: Potential issues and solutions

introduction

In the development of Java + MySQL, we usually use ORM frameworks (such as MyBatis, MyBatis-Plus, Hibernate) to map database tables and Java objects. But sometimes, after the database table structure changes (such as adding new fields), developers may forget to update the Java entity class synchronously. What problems will this cause? Especially when the program callssaveBatch()Will an error be reported when waiting for batch operation methods?

This article will conduct in-depth analysis from the following aspects:

• Problem background: The impact of MySQL adding fields, Java entities not being updated
• The impact of different operations (query, insert, batch insert)
• Solutions (temporary repairs vs. long-term best practices)
• Code examples and optimization suggestions

1. Problem background: The database is not synchronized with Java entities

1.1 Common Scenarios

• Added fields in the database (such asALTER TABLE ADD COLUMN), but the Java entity class is not updated.
• The program continues to run and callsaveBatch()、insert(), query and other methods.
• Will an error be reported? Depends on field constraints and ORM framework behavior.

1.2 Sample Code

Suppose there is aStatisticsDataEntity class (using MyBatis-Plus):

@Data
@TableName("statistics_data")
public class StatisticsData extends BaseModel {
    private String agentId;
    private Long click;
    // Other fields...}

Then add a new field to the database:

ALTER TABLE statistics_data ADD COLUMN new_column INT NOT NULL;

At this point, what impact will it have if the Java code is not updated?

2. Analysis of the impact of different operations

2.1 Query Operation (SELECT)

• By default, MyBatis-Plus ignores fields that exist in the database but do not have the entity class, and the query will not report an error.
• But if usingSELECT *Or manually map all fields, warnings may be triggered (depending on the log level).

2.2 Insert operation (INSERT)

If new fields are allowedNULLOr there is a default value:

ALTER TABLE statistics_data ADD COLUMN new_column INT DEFAULT 0;

• ✅ save()orsaveBatch()There will be no error, this field will be used when insertingNULLor the default value fill.

If the new field isNOT NULLAnd no default value:

ALTER TABLE statistics_data ADD COLUMN new_column INT NOT NULL;

❌ saveBatch()An error will be reported:

ERROR 1364 (HY000): Field 'new_column' doesn't have a default value

MySQL refuses to insert because the SQL generated by MyBatis-Plus does not contain undefined fields.

2.3 Batch insertion (saveBatch)

saveBatch()The underlying logic:

// MyBatis-Plus default implementation (simplified version)public boolean saveBatch(Collection<T> entityList) {
    for (T entity : entityList) {
        (entity); // Generate INSERT SQL, containing only fields defined by entity class    }
    return true;
}

• ifnew_columnyesNOT NULL, MySQL will report an error because SQL does not contain this field.
• If allowedNULLOr set the default value and execute normally.

3. Solution

3.1 Temporary repair (not recommended for a long time)

(1) Modify database field constraints

-- allow NULL
ALTER TABLE statistics_data MODIFY new_column INT NULL;
-- Or set the default value
ALTER TABLE statistics_data MODIFY new_column INT DEFAULT 0;

(2) Avoid automatic mapping and manually specify SQL

// Ignore unknown fields with @TableField(exist = false)@TableField(exist = false)
private String ignoredField;
// Or custom SQL (expressly specify the insert field)@Insert("INSERT INTO statistics_data (agent_id, click) VALUES (#{agentId}, #{click})")
void customInsert(StatisticsData data);

3.2 Long-term best practices (recommended)

(1) Synchronously update Java entity class

@Data
@TableName("statistics_data")
public class StatisticsData extends BaseModel {
    private String agentId;
    private Long click;
    private Integer newColumn; // Add new fields}

(2) Use database migration tools (such as Flyway/Liquibase)

-- V1__init.sql
CREATE TABLE statistics_data (...);
-- V2__add_new_column.sql
ALTER TABLE statistics_data ADD COLUMN new_column INT DEFAULT 0;

(3) Automatic inspection (optional)

Make sure the database is consistent with the entity class through unit testing or Schema verification tools:

// Example: Check with Hibernate Validator (if applicable)@Column(nullable = false)
private Integer newColumn;

4. Complete code example

4.1 Updated Java Entity

@Data
@TableName("statistics_data")
public class StatisticsData extends BaseModel {
    private String agentId;
    private Long click;
    private Integer newColumn; // Add new fields    @TableField("`date`")
    private String date;
    // Other fields...}

4.2 Safe batch insertion method

// Check the data integrity before insertingpublic void safeBatchInsert(List<StatisticsData> dataList) {
    if (dataList == null || ()) {
        return;
    }
    // Field verification can be done here    (dataList);
}

4.3 Database Change Script (Flyway Example)

-- V2__add_new_column.sql
ALTER TABLE statistics_data 
ADD COLUMN new_column INT NOT NULL DEFAULT 0 COMMENT 'Added field';

5. Summary

Best Practices:

• Keep the database and Java entities synchronized to avoid unknown problems.
• Use database migration tools such as Flyway to manage table structure changes.
• Set reasonable constraints on important fields (such asNOT NULL + DEFAULT）。

6. Expanding thinking

• If JPA/Hibernate is used, the table structure will be automatically checked when starting, and an error will be reported directly if it is inconsistent.
• MyBatis-Plus@TableField(exist = false)Non-database fields can be tagged.
• Automated generation of entity-like tools such as MyBatis-Plus Generator reduces manual synchronization costs.

Through reasonable architectural design, this kind of problem of "database and code are out of sync" can be avoided and system stability can be improved.

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