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Tooling beginner

Single-file execution

Run single-file Java programs directly without javac.

Code Comparison
✕ Java 8 
$ javac HelloWorld.java
$ java HelloWorld
// Two steps every time
✓ Java 11+ 
$ java HelloWorld.java
// Compiles and runs in one step
// Also works with shebangs:
#!/usr/bin/java --source 25
Why the modern way wins
⚡

One command

java File.java compiles and runs in one step.

📝

Script-like

Add a shebang line to make .java files executable scripts.

🎓

Learning-friendly

Newcomers run code immediately without learning build tools.

Old Approach
Two-Step Compile
Modern Approach
Direct Launch
Since JDK
11
Difficulty
beginner
JDK Support
Single-file execution
Available

Widely available since JDK 11 (Sept 2018)

How it works

The Java launcher can compile and run a single source file in one command. Combined with shebang support on Unix, Java files can work as scripts. No separate compilation step needed.

Related patterns

Tooling intermediate

Multi-file source launcher

Java 8
$ javac *.java
$ java Main
// Must compile all files first
// Need a build tool for dependencies
Java 22+
$ java Main.java
// Automatically finds and compiles
// other source files referenced
// by Main.java
Hover to see modern ➜
JDK 22+ →
Tooling advanced

Compact object headers

Java 8
// Default: 128-bit object header
// = 16 bytes overhead per object
// A boolean field object = 32 bytes!
// Mark word (64) + Klass pointer (64)
Java 25
// -XX:+UseCompactObjectHeaders
// 64-bit object header
// = 8 bytes overhead per object
// 50% less header memory
// More objects fit in cache
Hover to see modern ➜
JDK 25+ →
Tooling intermediate

JFR for profiling

Java 8
// Install VisualVM / YourKit / JProfiler
// Attach to running process
// Configure sampling
// Export and analyze
// External tool required
Java 9+
// Start with profiling enabled
$ java -XX:StartFlightRecording=
    filename=rec.jfr MyApp

// Or attach to running app:
$ jcmd <pid> JFR.start
Hover to see modern ➜
JDK 9+ →
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