Introduction

Java ConcurrentModificationException occurs when collection is modified during iteration. This guide provides step-by-step diagnosis and resolution with specific commands and code examples.

Symptoms

Typical symptoms and error messages when this issue occurs:

bash
java.lang.Error: Unexpected error occurred
	at com.example.Application.main(Application.java:42)
Caused by: internal error

Observable indicators: - Application logs show errors or exceptions - JVM crashes or becomes unresponsive - Related services may fail or timeout

Common Causes

  1. 1.Thread concurrency issues stem from:
  2. 2.Improper synchronization primitives usage
  3. 3.Race conditions in shared state access
  4. 4.Missing thread safety in collections
  5. 5.Incorrect thread pool configuration

Step-by-Step Fix

Step 1: Check Current State

bash
java -version

Step 2: Identify Root Cause

bash
jcmd <pid> VM.info

Step 3: Apply Primary Fix

java
// Primary fix: update configuration
@Configuration
public class AppConfig {
    @Bean
    public MyBean myBean() {
        MyBean bean = new MyBean();
        bean.setTimeout(30000);
        return bean;
    }
}

Apply this configuration and restart the application.

Step 4: Apply Alternative Fix (If Needed)

java
// Alternative fix: use properties
# application.properties
app.timeout=30000
app.retry-count=3
app.enabled=true

Run load tests to verify thread safety under concurrent access.

Step 5: Verify the Fix

After applying the fix, verify with:

bash
java -jar application.jar --debug

Expected output should show successful operation without errors.

Common Pitfalls

  • Not reading error messages carefully
  • Applying fixes without understanding root cause
  • Skipping verification after changes

Best Practices

  • Read official documentation first
  • Test in isolated environment
  • Document changes for team visibility
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