JDK vs JRE

🔍 What Is It?

JRE — Java Runtime Environment

Contains:

• →JVM (the engine that runs bytecode)
• →Java Class Library (java.lang, java.util, java.io, etc.)
• →Supporting files and configuration

Used by: end users running Java applications, production servers (historically)

JDK — Java Development Kit

Contains everything in JRE plus:

• →javac — Java compiler
• →javadoc — documentation generator
• →jar — archive tool
• →jdb — Java debugger
• →jshell — interactive REPL (Java 9+)
• →jconsole, jvisualvm — monitoring tools
• →jstack, jmap, jstat — diagnostic tools

Used by: developers, CI/CD pipelines, build servers

JDK Distributions (2025)

• →OpenJDK — open-source reference implementation (free)
• →Eclipse Temurin (Adoptium) — most popular free distribution for production
• →Amazon Corretto — AWS-optimized, free, LTS patches
• →Microsoft Build of OpenJDK — Azure-optimized
• →Oracle JDK — commercial license required for production (Java 17+)
• →GraalVM — includes native image compiler
• →Azul Zulu — certified builds with extended support

🎯 Why Do We Need It?

Understanding JDK vs JRE matters for:

• →Docker images — use JRE-based images for production (smaller), JDK for build stages
• →CI/CD — build agents need JDK; runtime containers need JRE
• →License compliance — Oracle JDK requires a commercial license; use Temurin/Corretto for free
• →Version management — enterprise projects often have multiple Java versions; tools like SDKMAN manage this

🏗️ What Happens in Real Projects?

In a real enterprise Dockerfile (multi-stage build):

# Stage 1: Build with JDK
FROM eclipse-temurin:21-jdk AS builder
WORKDIR /app
COPY . .
RUN ./mvnw package -DskipTests

# Stage 2: Run with JRE (smaller image)
FROM eclipse-temurin:21-jre
WORKDIR /app
COPY --from=builder /app/target/app.jar .
CMD ["java", "-jar", "app.jar"]

The JRE image is typically 100–150MB smaller than the JDK image.

⚙️ How It Works

Since Java 11, Oracle stopped shipping a standalone JRE. The JDK is now used everywhere. However, you can create a custom minimal runtime using jlink:

# Create a minimal JRE containing only the modules your app needs
jlink --module-path $JAVA_HOME/jmods \
  --add-modules java.base,java.logging,java.sql \
  --output my-custom-jre

This produces a runtime as small as 30MB, much smaller than a full JDK.

Version Management with SDKMAN:

sdk install java 21.0.1-tem   # Eclipse Temurin 21
sdk install java 17.0.9-tem   # Eclipse Temurin 17
sdk use java 17.0.9-tem       # switch for current session
sdk default java 21.0.1-tem   # set default

💻 Example

JDK Tools in Practice — The JDK tools you will use daily as a backend developer.

# Check installed Java version
java -version
javac -version

# jshell — interactive Java REPL (great for testing snippets)
jshell
jshell> var list = List.of(1, 2, 3);
jshell> list.stream().mapToInt(x -> x).sum()
$2 ==> 6

# jar — inspect a JAR file
jar tf myapp.jar           # list contents
jar xf myapp.jar           # extract

# javap — disassemble bytecode
javap -c MyClass.class
javap -verbose MyClass.class

# jdeps — analyze dependencies
jdeps --print-module-deps myapp.jar

# jlink — create minimal custom JRE
jlink --add-modules java.base,java.net.http \
  --output custom-jre --strip-debug --compress=2

🤔 Common Confusions

🚨 Common Production Issues

✅ Best Practices

🏛️ Architect's Note

💬 Frequently Asked Questions

🎯 Key Takeaways

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