java.lang.NoClassDefFoundError: com/example/MissingClass

What it means

NoClassDefFoundError is thrown when the JVM successfully resolved a class at compile time but cannot produce a usable definition of it now. That is a different statement from ClassNotFoundException, which means nobody could find the class at all, and the difference determines where to look.

The case that makes this error genuinely confusing on Lambda is static initialisation. A class's static block and static field initialisers run exactly once, the first time the class is touched. If any of that throws, the JVM marks the class erroneous — permanently, for the lifetime of that JVM. The first failure surfaces as ExceptionInInitializerError and carries the real exception with a real stack trace. Every subsequent use of that class, for as long as the execution environment lives, throws NoClassDefFoundError: Could not initialize class … with no indication of the original cause.

On Lambda this interacts badly with execution-environment reuse. One container serves many invocations. The class is poisoned on invocation 1, and invocations 2 through 200 on that same container all fail with the uninformative echo — each under its own request ID, each looking like an independent failure. Meanwhile a different container that never took that code path keeps succeeding, so the function appears to fail intermittently at a stable rate. It is not intermittent: it is deterministic per container.

That is also why this is one of the errors most worth reading against a whole log stream rather than a single invocation. The invocation you are looking at contains the echo. The invocation that contains the answer happened earlier, under a request ID you have no reason to search for, and the only thing connecting them is that they ran in the same execution environment.

What it looks like in CloudWatch

This is the shape the failure arrives in: the lines of one invocation scattered among everything else the log group received at the same moment.

The panel on the right is generated by running the excerpt on the left through the same parser that powers the free web stitcher — it is what the tool actually produces for this input, not an illustration of it.

How to confirm it from the logs

The wording separates the two cases, and it is worth reading carefully. A bare NoClassDefFoundError: com/example/Thing (note the slashes — this is an internal class name, not a package name) usually means the class is genuinely absent. Could not initialize class com.example.Thing means the opposite: it was found, and its static setup failed.

For the second case, look earlier in the same invocation, or in an earlier invocation on the same container. The real exception — the one the static initialiser threw — was logged the first time the class was touched. Every subsequent attempt produces this far less informative error instead. If you only look at the failing invocation you will never see the cause, because it was reported minutes earlier under a different request ID.

Causes, most likely first

Fixes

bash# Look for the original failure, not the echo.
aws logs filter-log-events \
  --log-group-name /aws/lambda/order-function \
  --filter-pattern 'ExceptionInInitializerError' \
  --start-time $(( ($(date +%s) - 3600) * 1000 ))

java// Risky: throws at class load, poisons the class for the whole container.
// private static final String TABLE = System.getenv("TABLE").trim();

// Safer: fails per invocation, with a message that says what is wrong.
private static String table() {
  String value = System.getenv("TABLE");
  if (value == null) {
    throw new IllegalStateException("TABLE environment variable is not set");
  }
  return value;
}

bashmvn clean package shade:shade
unzip -l target/function.jar | grep -c '\.class$'

Also seen as

The same underlying failure, worded differently by a different runtime, SDK version, or logging layer. All of these land here — there is no separate page for each phrasing.

Related errors

Errors that show up alongside this one, or that people mistake for it.

References

• AWS Lambda Developer Guide — Define Lambda function handler in Java
• Java SE API — NoClassDefFoundError