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common-apex-runtime-errors

Diagnosing and resolving common Apex runtime exceptions: NullPointerException, QueryException, DmlException, ListException, LimitException, TypeException. Use when debugging Apex runtime failures or writing defensive code. NOT for structuring try/catch — use apex/exception-handling. NOT for a cross-cutting error framework — use apex/error-handling-framework. NOT for governor-limit prevention — use apex/governor-limits.

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Common Apex Runtime Errors

Use this skill when an Apex class or trigger throws a runtime exception and you need to identify the root cause and apply the correct fix. It covers the six most common exception types — NullPointerException, QueryException, DmlException, ListException, LimitException, and TypeException/StringException — each with diagnosis path and resolution pattern.

Before Starting

Gather this context before working on anything in this domain:

Core Concepts

The Apex runtime throws built-in exceptions that are subclasses of System.Exception. Most are catchable in a try/catch block — but LimitException is explicitly uncatchable and must be prevented upstream. Each exception class carries a distinct message format that maps directly to a root cause.

NullPointerException

Thrown when code dereferences a variable that is null. Common triggers:

Resolution: Always check for null before accessing members. For SOQL scalars, wrap in a try/catch for QueryException or use a List and check .isEmpty(). For map lookups, use map.containsKey(key) before calling .get().

QueryException

Thrown in two scenarios:

  1. Too few rows: A SOQL query assigned directly to a scalar SObject variable returns zero rows. Message: List has no rows for assignment to SObject.
  2. Too many rows: The same scalar assignment returns two or more rows. Message: List has more than 1 row for assignment to SObject.

Resolution: Use a List<SObject> instead of a scalar when the result set size is uncertain. Check list.isEmpty() before accessing list[0]. When exactly one row is expected and a miss is a true error, catch QueryException explicitly and surface a meaningful error rather than letting the raw exception propagate.

DmlException

Thrown when a DML operation (insert, update, delete, upsert) fails at the database level. Common causes:

Note: mixing an insert of two non-setup types (e.g. an Account and a Contact) into separate DML statements in one transaction is fine — that is not what triggers a mixed-DML failure. The mixed-DML restriction is specifically about setup vs non-setup objects; see Mixed DML (MIXED_DML_OPERATION) below.

The DmlException is the only common exception that carries per-row error details. Use e.getNumDml(), e.getDmlMessage(i), e.getDmlIndex(i), and e.getDmlFields(i) to extract which records failed and why.

Resolution: Validate required fields before DML. Use Database.insert/update/delete with allOrNone=false and inspect Database.SaveResult[] for partial-success scenarios. Log per-row getDmlMessage errors rather than only the top-level exception message.

Mixed DML — System.DmlException with status code MIXED_DML_OPERATION

There is no MixedDmlException type in Apex. This is one of the most common wrong identifiers in the language. catch (MixedDmlException e) does not compile, and grepping production logs for System.MixedDMLException returns nothing. Mixed setup/non-setup DML raises an ordinary System.DmlException carrying the status code MIXED_DML_OPERATION — a real member of the API StatusCode enumeration. The log line looks like:

System.DmlException: Insert failed. First exception on row 0; first error:
MIXED_DML_OPERATION, DML operation on setup object is not permitted after you
have updated a non-setup object (or vice versa): []

Catch it as a DmlException and discriminate on the message or on e.getDmlType(0), not on a dedicated exception class.

It occurs when a DML operation on a setup object is mixed with a DML operation on a non-setup object in the same transaction. Per the official docs: "DML operations on certain sObjects, sometimes referred to as setup objects, can't be mixed with DML on non-setup sObjects in the same transaction." The restriction exists because some setup objects affect the running user's access to records.

The failure is not caused by mixing two arbitrary non-setup types. It fires only when a setup-object DML and a non-setup-object DML land in the same transaction.

Resolution: Run the setup-object DML in its own transaction. The standard fix is to move it into an asynchronous context — a @future method or a Queueable — so it executes in a separate transaction from the non-setup DML. In test code, System.runAs() blocks and async jobs called by the test are permitted to perform mixed DML.

Row-lock contention — "Unable to lock row"

A distinct, very common DmlException variant: Unable to lock row - Record currently unavailable. Salesforce locks records during updates; a transaction can wait a maximum of 10 seconds for a lock to be released before timing out and throwing this error. Frequent sources are concurrent DML on the same records, long-running Apex triggers, Bulk API loads running in parallel mode, and Master-Detail child updates that contend for the same parent lock.

Resolution: Reduce trigger execution time and avoid explicit FOR UPDATE locking statements in triggers. For Bulk API loads, reduce batch size, switch from parallel to serial mode, or sort each batch by parent record ID so batches don't contend for the same parent locks. For Master-Detail, distribute child records across parents rather than concentrating them. Enable debug logs for the affected user to identify the offending trigger, flow, or validation rule.

SObjectException

A built-in exception raised for problems with sObject records — most commonly attempting to change a field in an update statement that can only be set during insert, or accessing a field that was not included in the SOQL query. The docs describe it as covering "any problem with sObject records, such as attempting to change a field in an update statement that can only be changed during insert."

Resolution: Only write to fields that are updateable in the current DML context — never assign to read-only, formula, or insert-only fields on an existing record. Include every field you intend to read in the SOQL SELECT clause. This is one of many named built-in exceptions in Apex (the Apex Reference lists dozens, including CalloutException, JSONException, SecurityException, and TypeException); the six headline types in this skill are the most frequently hit, and SObjectException is a common near-neighbor worth recognizing on sight.

ListException

Thrown when code accesses a List index that does not exist. Message: List index out of bounds: N. Common causes:

Resolution: Check list.size() > index or !list.isEmpty() before indexed access. Prefer for (SObject o : list) iteration over manual index loops.

LimitException — UNCATCHABLE

Thrown when a governor limit is exceeded: CPU time, SOQL queries, DML rows, heap size, callouts, and so on. Message examples: Too many SOQL queries: 101, Apex CPU time limit exceeded.

LimitException cannot be caught with try/catch. Any catch (LimitException e) block will never execute. The transaction is terminated by the platform immediately.

Resolution: Use Limits.getQueries() / Limits.getLimitQueries() guards before issuing SOQL inside loops. Bulkify triggers. Move heavy work to Queueable or Batch Apex. The governor-limits skill covers prevention patterns in depth.

TypeException and StringException

TypeException is thrown when an explicit cast fails (e.g. casting an Integer to a Date) or when JSON.deserialize produces a type mismatch. StringException is thrown by String.format() when the argument count does not match the placeholder count.

Resolution: Validate input types before casting. Use instanceof for runtime type checks. Prefer JSON.deserializeUntyped() with manual type checks over strongly-typed deserialization when the input shape is uncertain.

Common Patterns

Pattern: SOQL-safe scalar assignment

When to use: Any time you expect exactly one row from a SOQL query but the data could theoretically return zero or multiple rows (i.e., almost always).

How it works:

List<Account> accounts = [SELECT Id, Name FROM Account WHERE Id = :recordId LIMIT 1];
if (accounts.isEmpty()) {
    throw new AuraHandledException('Account not found: ' + recordId);
}
Account acc = accounts[0];

Why not the alternative: Direct scalar assignment Account acc = [SELECT Id FROM Account WHERE Id = :recordId] throws QueryException on zero rows and on two or more rows. Both failure modes are common in real orgs where data is inconsistent.

Pattern: DML with per-row error capture

When to use: Bulk DML operations where partial success is acceptable and each failed row must be logged.

How it works:

List<Database.SaveResult> results = Database.insert(records, false);
for (Integer i = 0; i < results.size(); i++) {
    if (!results[i].isSuccess()) {
        for (Database.Error err : results[i].getErrors()) {
            System.debug('Row ' + i + ' failed: ' + err.getMessage()
                + ' Fields: ' + err.getFields());
        }
    }
}

Why not the alternative: insert records; with allOrNone=true (the default) rolls back the entire batch on a single-row validation failure, which is rarely the right behavior for bulk operations.

Pattern: LimitException prevention guard

When to use: Any trigger or service method that issues SOQL inside a loop or recursively.

How it works:

if (Limits.getQueries() + 1 >= Limits.getLimitQueries()) {
    // Log and abort gracefully — do NOT attempt the query
    System.debug(LoggingLevel.ERROR, 'SOQL limit near — aborting query batch');
    return;
}
List<Contact> contacts = [SELECT Id FROM Contact WHERE AccountId IN :accountIds];

Why not the alternative: You cannot catch LimitException. If the limit is crossed, the transaction terminates with no opportunity to log or recover.

Decision Guidance

ExceptionRoot CauseResolution
NullPointerException on SObject fieldSOQL returned 0 rows into scalar variableUse List + isEmpty() check
NullPointerException on method chainMethod returned null, caller chained without guardAdd null check before chaining
QueryException: List has no rowsScalar SOQL on zero-row resultUse List or catch QueryException
QueryException: List has more than 1 rowScalar SOQL matched multiple rowsAdd WHERE filters or use LIMIT 1
DmlException: required field missingField blank on record before DMLValidate fields before insert/update
DmlException: duplicate valueDuplicate rule or unique constraint hitCheck for existing record first or use upsert
DmlException: Unable to lock rowRow-lock contention (concurrent DML, long triggers, Bulk API parallel mode, MD child updates)Shorten triggers; Bulk API serial mode + batch sort by parent ID; distribute MD children
DmlException: MIXED_DML_OPERATIONSetup-object DML (User, Group, PermissionSet…) mixed with non-setup DML in one transaction. Note: there is no MixedDmlException type — catch DmlExceptionDefer setup-object DML to @future / Queueable so it runs in its own transaction
SObjectExceptionWriting an insert-only/read-only field on update, or reading a field not in the SELECTOnly update updateable fields; add the field to the SOQL SELECT
ListException: index out of boundsIndexed access on empty or short listCheck list.size() before access
LimitException: Too many SOQL queriesSOQL inside loopBulkify: move SOQL outside loop
LimitException: CPU time exceededNested loops or heavy string ops on large setsMove work to Batch Apex
TypeExceptionInvalid cast or JSON type mismatchUse instanceof check before cast
StringExceptionString.format argument count mismatchMatch placeholder count to argument list

Step-by-step instructions for an AI agent or practitioner diagnosing an Apex runtime exception:

  1. Open the debug log or test failure stack trace. Locate the FATAL_ERROR or EXCEPTION_THROWN event line. Note the exception class, message text, and line number.
  2. Match the exception class to the six types in this skill (NullPointerException, QueryException, DmlException, ListException, LimitException, TypeException/StringException). If the class is not one of these, escalate to the exception-handling skill.
  3. For NullPointerException: identify the null source — SOQL scalar result, method return value, or map lookup. Apply null guard or switch to List-based SOQL pattern.
  4. For QueryException: switch scalar SOQL assignment to List<SObject> and add isEmpty() guard. For DmlException: add Database.insert/update/delete with allOrNone=false and log per-row errors using getDmlMessage(i).
  5. For LimitException: confirm via Limits.* API whether the limit is being approached. Bulkify the operation. If the fix requires architectural changes (e.g. moving to Batch Apex), flag for design review.
  6. Apply defensive code pattern from the relevant section above. Add or update a unit test that exercises the failure path (zero-row SOQL, DML validation failure, boundary index access) to confirm the fix holds under real data conditions.
  7. Run python3 skills/apex/common-apex-runtime-errors/scripts/check_common_apex_runtime_errors.py --manifest-dir force-app/main/default/classes to scan for unguarded patterns in the codebase.

Review Checklist

Run through these before marking work in this area complete:

Salesforce-Specific Gotchas

Non-obvious platform behaviors that cause real production problems:

  1. LimitException is uncatchable — Unlike every other built-in exception, System.LimitException cannot be caught in a try/catch block. Code that wraps DML or SOQL in try/catch and expects to catch a limit breach will silently fail to catch it. The transaction terminates without entering any catch block.
  2. Null SOQL scalar vs. empty List — Account a = [SELECT Id FROM Account WHERE Id = :id] returns null when zero rows match, but a List<Account> query returns an empty list. Many developers expect the scalar form to throw QueryException immediately, but the exception is deferred to the first field access on the null reference, making stack traces misleading.
  3. DmlException row index mismatch after partial-success — When using Database.insert(records, false), the SaveResult array index maps to the original records list, not a filtered list of failed records. Using getDmlIndex(i) on the exception (from allOrNone=true mode) is required to find the source record; iterating the result array directly gives you the right index only in allOrNone=false mode.

Output Artifacts

ArtifactDescription
Corrective code diffReplace unsafe SOQL scalar / unguarded DML / indexed list access with the safe pattern for the specific exception type
Defensive checklistPer-exception review checklist confirming all failure paths are covered
Checker reportOutput of check_common_apex_runtime_errors.py listing unguarded patterns in the scanned Apex class directory

Package files