Check Errors Skill

Identify error handling gaps, anti-patterns, and inconsistencies in exception management across the codebase.

Instructions

1. Find Swallowed Exceptions

Search for empty or inadequate catch blocks:

// BAD: Empty catch block
try {
    riskyOperation();
} catch (Exception e) {
    // silently swallowed!
}

// BAD: Only logging, not handling
try {
    riskyOperation();
} catch (Exception e) {
    log.error("Error", e);
    // continues as if nothing happened
}

// BAD: Catching and returning null
try {
    return fetchData();
} catch (Exception e) {
    return null;  // Caller doesn't know it failed
}

Use Grep to find:

# Empty catch blocks
catch.*\{[\s]*\}

# Catch with only comment
catch.*\{[\s]*/[/*]

# Catch with only log statement and no throw/return
catch.*\{[\s]*log\.(error|warn|info)

2. Check Reactive Error Handling

Missing error operators in Mono/Flux chains:

// BAD: No error handling
return webClient.get()
    .retrieve()
    .bodyToMono(Data.class);  // What if it fails?

// GOOD: Proper error handling
return webClient.get()
    .retrieve()
    .bodyToMono(Data.class)
    .onErrorResume(e -> {
        log.error("Failed to fetch", e);
        return Mono.empty();
    });

// GOOD: With fallback
return service.fetchData()
    .onErrorReturn(defaultValue);

// GOOD: Transform error
return service.fetchData()
    .onErrorMap(e -> new DomainException("Fetch failed", e));

Search for chains without error handling:

// Find Mono/Flux returns without onError*
\.bodyToMono\(.*\);$
\.bodyToFlux\(.*\);$
\.flatMap\(.*\);$  // without subsequent onError

3. Analyze Exception Propagation

Checked vs Unchecked exceptions:

// BAD: Wrapping checked in RuntimeException without context
try {
    Files.readAllBytes(path);
} catch (IOException e) {
    throw new RuntimeException(e);  // Loses context
}

// GOOD: Wrap with meaningful exception
try {
    Files.readAllBytes(path);
} catch (IOException e) {
    throw new ConfigLoadException("Failed to load config: " + path, e);
}

Search for generic exception wrapping:

throw new RuntimeException\(e\)
throw new RuntimeException\(.*e\)
throw new IllegalStateException\(e\)

4. Controller Error Handling

Check for @ExceptionHandler coverage:

// Global exception handler should exist
@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(NotFoundException.class)
    public ResponseEntity<ErrorResponse> handleNotFound(NotFoundException e) {
        return ResponseEntity.status(404).body(new ErrorResponse(e.getMessage()));
    }

    @ExceptionHandler(Exception.class)  // Catch-all
    public ResponseEntity<ErrorResponse> handleGeneral(Exception e) {
        log.error("Unexpected error", e);
        return ResponseEntity.status(500).body(new ErrorResponse("Internal error"));
    }
}

Verify:

• @ControllerAdvice exists
• Common exceptions are handled (NotFound, BadRequest, etc.)
• Catch-all handler exists
• Error responses are consistent

Check controllers for unhandled exceptions:

// BAD: Exception can escape to framework
@GetMapping("/{id}")
public Mono<Data> getById(@PathVariable int id) {
    return service.findById(id);  // What if not found?
}

// GOOD: Proper handling
@GetMapping("/{id}")
public Mono<ResponseEntity<Data>> getById(@PathVariable int id) {
    return service.findById(id)
        .map(ResponseEntity::ok)
        .defaultIfEmpty(ResponseEntity.notFound().build());
}

5. Error Response Consistency

Check error response format:

// All error responses should follow same structure
{
    "error": "Error message",
    "code": "ERROR_CODE",
    "timestamp": "2024-01-01T00:00:00Z",
    "path": "/api/resource"
}

Find inconsistent error responses:

// Different formats
ResponseEntity.badRequest().body("Error")           // String
ResponseEntity.badRequest().body(Map.of("msg", e))  // Map
ResponseEntity.badRequest().body(new Error(e))      // Object

6. Null Handling Gaps

Missing null checks:

// BAD: No null check before use
public void process(Data data) {
    String value = data.getValue().toUpperCase();  // NPE if null
}

// GOOD: Defensive coding
public void process(Data data) {
    if (data == null || data.getValue() == null) {
        throw new IllegalArgumentException("Data and value required");
    }
    String value = data.getValue().toUpperCase();
}

// GOOD: Optional usage
public void process(Data data) {
    Optional.ofNullable(data)
        .map(Data::getValue)
        .ifPresent(this::handleValue);
}

Search patterns:

# Method calls on potentially null returns
\.get\(\)\.           # get() followed by method call
\.findFirst\(\)\.get  # Optional.get() without isPresent

7. Resource Cleanup on Error

Check try-with-resources usage:

// BAD: Resource leak on exception
InputStream is = new FileInputStream(file);
try {
    process(is);
} finally {
    is.close();  // May throw, masking original exception
}

// GOOD: Try-with-resources
try (InputStream is = new FileInputStream(file)) {
    process(is);
}

Check OkHttp response body closure:

// BAD: Response body not closed on error
Response response = client.newCall(request).execute();
if (!response.isSuccessful()) {
    throw new ApiException("Failed");  // Body leaked!
}
String body = response.body().string();

// GOOD: Try-with-resources
try (Response response = client.newCall(request).execute()) {
    if (!response.isSuccessful()) {
        throw new ApiException("Failed: " + response.code());
    }
    return response.body().string();
}

8. Logging Quality

Check error logging:

// BAD: No stack trace
log.error("Error occurred");
log.error("Error: " + e.getMessage());

// BAD: Wrong log level
log.info("Error occurred", e);  // Should be error/warn

// GOOD: Full context
log.error("Failed to process request for user={}", userId, e);

Search for inadequate logging:

log\.error\("[^"]*"\);$           # error without exception
log\.error\(".*" \+ e\.getMessage  # message only, no stack
catch.*\{[^}]*\}                   # catch without any log

9. Timeout and Retry Handling

Check for timeout handling:

// BAD: No timeout
Response response = client.newCall(request).execute();

// GOOD: Configured timeout
OkHttpClient client = new OkHttpClient.Builder()
    .connectTimeout(10, TimeUnit.SECONDS)
    .readTimeout(30, TimeUnit.SECONDS)
    .build();

// Check reactive timeouts
mono.timeout(Duration.ofSeconds(30))
    .onErrorResume(TimeoutException.class, e -> fallback());

Check for retry logic:

// With Spring Retry
@Retryable(value = ApiException.class, maxAttempts = 3)
public Data fetchData() { ... }

@Recover
public Data fallback(ApiException e) { ... }

// With Reactor
mono.retryWhen(Retry.backoff(3, Duration.ofSeconds(1)))

10. Project-Specific Checks

Files to examine:

• controller/*.java - HTTP error responses
• service/*.java - Business exception handling
• service/strategy/*.java - External API error handling
• config/*Config.java - Configuration error handling

This project's patterns to verify:

// AggregatorService - scheduled task error handling
// ForecastService - Windguru API error handling
// CurrentConditionsService - Weather station error handling
// GoogleMapsService - URL resolution error handling

Output Format

## Error Handling Analysis Report

### Summary
| Category | Issues | Severity |
|----------|--------|----------|
| Swallowed Exceptions | X | Critical |
| Missing Reactive Handlers | X | High |
| Inconsistent Error Responses | X | Medium |
| Resource Leaks on Error | X | High |
| Inadequate Logging | X | Medium |

### Critical Issues

#### Swallowed Exception
**File**: `path/to/file.java:line`
**Code**:
```java
try {
    externalService.call();
} catch (Exception e) {
    // empty
}

Risk: Silent failures, data inconsistency Fix:

try {
    externalService.call();
} catch (Exception e) {
    log.error("External service call failed", e);
    throw new ServiceException("Failed to call external service", e);
}

High Priority Issues

Missing Reactive Error Handler

File: path/to/file.java:line Chain: webClient.get()...bodyToMono() Risk: Unhandled errors propagate to framework Fix: Add .onErrorResume() or .onErrorReturn()

Resource Leak Risk

File: path/to/file.java:line Resource: OkHttp Response Fix: Wrap in try-with-resources

Medium Priority Issues

File	Line	Issue	Recommendation
Service.java	42	Generic RuntimeException wrap	Use domain exception
Handler.java	78	Log without stack trace	Add exception parameter

Error Handler Coverage

Exception Type	Handler Exists	Location
NotFoundException	✓	GlobalExceptionHandler:25
ValidationException	✗	Missing
Generic Exception	✓	GlobalExceptionHandler:45

Reactive Chains Analysis

File	Line	Chain	Error Handling
ForecastService	42	Mono	✓ onErrorResume
AggregatorService	78	Flux	✗ Missing

Recommendations

1. Critical: Add error handling to empty catch at File.java:42
2. High: Add @ControllerAdvice for ValidationException
3. Medium: Standardize error response format across controllers
4. Low: Improve error log messages with more context

Error Handling Checklist

• No empty catch blocks
• All reactive chains have error operators
• @ControllerAdvice covers common exceptions
• Error responses follow consistent format
• Resources closed properly on error paths
• Errors logged with stack traces
• Timeouts configured for external calls
• Retry logic for transient failures

## Execution Steps

1. Use `Grep` to find empty/inadequate catch blocks
2. Use `Grep` to find reactive chains without error handling
3. Check for @ControllerAdvice and @ExceptionHandler
4. Analyze error response formats in controllers
5. Find resource usages without try-with-resources
6. Check logging statements in catch blocks
7. Verify timeout and retry configurations
8. Generate comprehensive report

## Notes

- Some "swallowed" exceptions may be intentional (e.g., optional cleanup)
- Reactive error handling is critical - errors should never silently disappear
- This project uses @Retryable in some services - verify @Recover exists
- OkHttp responses MUST be closed, even on error paths
- Empty Mono/Flux is sometimes valid fallback, but should be logged