AwsJsonPrinter class

A LogPrinter that emits one JSON object per log record in a format suited to AWS CloudWatch Logs and AWS Lambda.

Each line is a self-contained JSON object with a level field (rather than GCP's severity) and CloudWatch-conventional level names (WARN and FATAL — matching the native AWS Lambda JSON log format and SLF4J/Log4j2 conventions). CloudWatch Logs Insights can parse these fields directly: fields @timestamp, level, message.

Note: AWS Lambda Powertools (a separate AWS-published library) uses Python's WARNING/CRITICAL. We match the native Lambda runtime log format instead, which is the more common convention across AWS services. If you need Powertools-style names, copy this class and swap the levelValue mapping.

Reserved-key precedence

The fields level, message, timestamp, logger, data, error, and stackTrace are reserved — these are written by hyper_logger itself. Context entries with any of these names are silently dropped during formatting so an accidental context: {'level': 'BUG'} cannot break a CloudWatch Insights query. (The drop is silent rather than logged so the hot path doesn't recurse — set onError on the surrounding printer if you want a signal.)

Anything else flows through context

User-supplied context fields are written at the JSON root, where CloudWatch Logs Insights auto-discovers them. Use this for requestId, traceId, userId, etc. — they become first-class fields you can query with Insights.

CloudWatch error visibility

For severity ERROR and FATAL, when both an error and a stackTrace are attached to the entry, the stack trace is also embedded in the message field. CloudWatch's text search and Error Insights work on the message content, so this makes exceptions show up without extra setup.

Level → level string mapping

LogLevel	CloudWatch level
trace	TRACE
debug	DEBUG
info	INFO
warning	WARN
error	ERROR
fatal	FATAL

For Google Cloud Logging see GcpJsonPrinter; for Azure Application Insights see AzureJsonPrinter.

Implemented types

• LogPrinter

Constructors

AwsJsonPrinter({LogOutput output = print})

const

Properties

contextKey → String?

When non-null, user context is nested under this key instead of being merged at the JSON root.

no setter, inherited

hashCode → int

The hash code for this object.

no setter, inherited

levelKey → String

JSON key under which the level value is written (e.g. 'severity', 'level', 'severityLevel').

no setter

output → LogOutput

Sink for formatted output. Defaults to print.

final, inherited

reservedKeys → Set<String>

Reserved JSON keys at the root of the emitted object.

no setter

runtimeType → Type

A representation of the runtime type of the object.

no setter, inherited

timestampKey → String

JSON key for the ISO-8601 timestamp.

no setter, inherited

Methods

dispose() → void

Releases any resources held by this printer (timers, file handles, network sockets, etc.).

inherited

format(LogEntry entry) → List<String>

Formats entry into a list of output lines (always exactly one element). The list shape mirrors ComposablePrinter.format for consistency across the printer surface.

inherited

levelValue(LogLevel level) → String

Value to write under levelKey for level.

log(LogEntry entry) → void

Emits entry to this printer's destination.

inherited

noSuchMethod(Invocation invocation) → dynamic

Invoked when a nonexistent method or property is accessed.

inherited

toString() → String

A string representation of this object.

inherited

Operators

operator ==(Object other) → bool

The equality operator.

inherited