How to Build Security Guardrails for LLM Apps in Java

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A technical guide published on DEV Community outlines key security boundaries in Java-based large language model applications, focusing on where untrusted text can enter and cause harm. The article identifies three main entry points for untrusted input: user-submitted text, retrieved content from document stores, and model-generated output passed back into code. It warns that prompt injection — where user input is mistaken by the model as instructions — is a critical vulnerability when text is concatenated directly into prompts. The guide introduces a SAFE pattern using explicit delimiters and system-level instructions to ensure the model treats user input strictly as data, never as commands. Additional defenses covered include input validation, output validation, and PII redaction, forming a security-forward capstone to a broader series on reliable LLM development in Java.

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