1Z0-829 Java Platform Overview and Packaging Practice Question
Which TWO are characteristics of a multi-release JAR (MR-JAR)?
⚠ Common exam trap
A common mix-up: candidates confuse the version directory naming convention, assuming it uses a full version string like '9.0.4' instead of the correct major version number (e.g., '9'), or they mistakenly think an MR-JAR can only hold one version of each class.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The root of the JAR contains classes for the oldest supported version.
In a multi-release JAR (MR-JAR), the root of the JAR contains the classes compiled for the oldest supported Java version. This ensures backward compatibility: when the JAR is run on an older JVM that does not understand the META-INF/versions directory, it will use the classes from the root. The JVM automatically selects the appropriate versioned class from the META-INF/versions/<version>/ directory based on the major version of the running Java runtime.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The root of the JAR contains classes for the oldest supported version.
Why this is correct
Root provides the base version for platforms that don't support MR-JAR.
- ✗
The version directory must be named with the full version string like "9.0.4".
Why it's wrong here
It uses the major version number only, e.g., 9.
- ✓
It uses the META-INF/versions/ directory structure.
Why this is correct
Standard structure for versioned classes.
- ✗
It can only contain a single version of each class.
Why it's wrong here
MR-JAR can contain multiple versioned copies of the same class.
- ✗
It is created using the jlink tool.
Why it's wrong here
jlink creates runtime images, not MR-JARs.
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