PLCcom

First steps PLCcom MQTT Core for Java

The PLCcom MQTT Core module is the shared technical foundation used by the PLCcom MQTT Client and PLCcom MQTT Broker modules. It contains common limits, base exceptions, PKI helpers, result quality support and internal protocol code that keeps MQTT behavior consistent across both delivered libraries.

For most applications: do not depend on Core directly. Add plccom-mqtt-client, plccom-mqtt-broker, or both. Maven will bring plccom-mqtt-core automatically. Core is documented so developers can understand shared result objects, diagnostics, PKI handling and common exception behavior that appear in Client and Broker APIs.

PLCcom MQTT Core overview with shared result, diagnostics, PKI and protocol support used by Client and Broker.

How to read this API

Core is the shared foundation below the customer-facing Client and Broker libraries. It is intentionally smaller than the Client and Broker Javadocs. The classes documented here are the shared types that application developers may see when they handle results, diagnostics, PKI stores, TLS certificate decisions, base exceptions or common protocol limits. Protocol packet internals remain implementation detail and are not part of the supported SDK contract.

Shared behavior
Client and Broker use the same result quality model, base exception hierarchy and diagnostic event objects. This makes error handling feel consistent across the SDK.

Security foundation
The folder-based PKI store, own certificate handling and remote certificate validation contracts live in Core so Client and Broker follow the same PLCcom trust-store pattern.

Protocol support
Core owns common MQTT limits and internal protocol support. Only the stable shared API is documented; wire parsing and state machines remain intentionally hidden.

Main entry points

Entry point	Use it when	Typical source
`OperationResult`	You want PLCcom-style quality, localized text and optional inner exception information from higher-level results.	Returned by Client and Broker result classes.
`OperationResult.eQuality`	You need a stable machine-readable result state such as good, timeout, protocol error, authorization failure or certificate problem.	Available through every PLCcom MQTT operation result.
`PlccomMqttException`	You want to catch broad MQTT SDK errors without handling each specialized exception separately.	Thrown by Client and Broker APIs.
`PlccomMqttPkiStore`	You manage trusted, rejected, issuer and own certificates in the PLCcom folder layout.	Used by TLS options and certificate management tooling.
`PlccomMqttDiagnosticEvent`	You want structured runtime warnings and errors without binding the SDK to a logging framework.	Emitted through Client and Broker diagnostic listeners.
`PlccomMqttLimits`	You configure shared MQTT size and protocol bounds.	Used by advanced Client/Broker configuration.

Maven dependency

Most applications should not need this dependency explicitly. If you intentionally use shared Core API types, use the same v2 release line as Client and Broker:


<dependency>
    <groupId>com.indi-an.plccom</groupId>
    <artifactId>plccom-mqtt-core</artifactId>
    <version>2.x.x</version>
</dependency>

Replace 2.x.x with the concrete PLCcom MQTT v2 release version you use.

What belongs into the Core Javadoc?

The Core Javadoc describes only supported API surface that application developers may see through Client or Broker usage. It does not expose the internal packet parser, wire codec, flow-state helpers or session internals as customer contracts.

Included in Core Javadoc	Excluded from Core Javadoc
Shared public base exceptions used by Client and Broker APIs.	Classes below `com.plccom.mqtt.core.internal`.
Shared public limits, result base types and quality values used by the delivered APIs.	MQTT packet body parsers, property codecs, internal topic alias tables, QoS state machines and retained/session implementation helpers.
PKI store and own-certificate APIs that are intended for TLS trust management.	License-gate internals and other implementation-only support classes.

Why the Core Javadoc is intentionally small

A Javadoc page creates expectations. If an internal wire class appears in the public documentation, developers may reasonably assume that it is a supported extension point. For Core this would be misleading: the protocol helpers are allowed to change when MQTT conformance, interoperability or robustness require it. The stable customer-facing APIs live in the Client and Broker modules.

Rule: public Java visibility is not the same as supported SDK API. The release Javadocs document the supported API. Internal packages are excluded even when individual implementation classes are technically public for module or test integration reasons.

Useful Core concepts

Concept	Why it matters
Operation result and quality	Client and Broker result objects share a PLCcom-style quality model so applications can handle success and failure consistently.
Localized result text	Quality values can be turned into readable English or German messages for tools, diagnostics and customer-facing status displays.
PKI store	The folder-based trust store follows the PLCcom family pattern with own, trusted, issuer and rejected certificate areas.
Certificate validation events	Rejected or newly seen certificates can be surfaced to applications and tools so operators can trust or reject them deliberately.
Base exceptions	Client and Broker exceptions share common ancestry for broader error handling.
Diagnostic events	Network, certificate, protocol and callback problems can be reported as structured events with severity information.
Protocol limits	Shared bounds keep MQTT packet size, string and binary handling consistent.

When should you look at Core?

Situation	What Core explains
A Client or Broker method returns a result object.	The common quality model, result text and inner exception pattern.
A TLS connection places a certificate in `rejected`.	The PLCcom PKI folder structure and certificate trust workflow.
A diagnostic listener receives a warning.	The event category, severity, component and operation fields.
You catch a broad MQTT SDK exception.	The common exception base and specialized protocol, timeout, TLS or authentication subclasses.

Recommended developer path

Goal	Use this module first
Connect to an existing broker, publish and subscribe.	PLCcom MQTT Client
Run a broker inside your Java process.	PLCcom MQTT Broker
Share common exception, PKI, result or limit handling between advanced integrations.	PLCcom MQTT Core

Back to PLCcom MQTT SDK overview.

Packages
Package	Description
com.plccom.mqtt
com.plccom.mqtt.client
com.plccom.mqtt.core
com.plccom.mqtt.diagnostics
com.plccom.mqtt.pki

PLCcom MQTT Core Java class library documentation