Java New Added 10 September 2026

Java: Cannot invoke "String.length()" because "s" is null

This is a helpful NullPointerException message, on by default since Java 15, and it names the exact expression that was null rather than just the line. When the name reads as <local1> or a synthetic name, the class was compiled without debug information, so recompiling with -g restores the real variable name.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

# Read the message carefully: it names the null part of the chain
# "because the return value of "Map.get(Object)" is null" means the key was missing

# Keep variable names in the message
javac -g:vars -d out src/**/*.java
# Maven: <maven.compiler.debug>true</maven.compiler.debug> (the default)

// Fix the source of the null, not the symptom
Optional.ofNullable(map.get(key))
        .map(String::trim)
        .orElseThrow(() -> new IllegalStateException("no entry for " + key));

// Fail early at the boundary
Objects.requireNonNull(config.url(), "config.url");

// If the JVM prints the old bare message, the flag was turned off
java -XX:+ShowCodeDetailsInExceptionMessages -jar app.jar

How to diagnose Java errors

Java errors concentrate at the class loading boundary (ClassNotFoundException, NoClassDefFoundError, UnsupportedClassVersionError) and around resource pools under load. Class loading errors are almost always classpath or version problems rather than missing code. UnsupportedClassVersionError in particular is a pure bytecode-version mismatch and tells you exactly which JDK compiled the class.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Print the actual runtime version with java -version and compare it against your build target. Major bytecode version 65 is Java 21, 61 is Java 17, 52 is Java 8.
Inspect the resolved dependency tree (mvn dependency:tree, gradle dependencies) to find duplicate or conflicting versions of the same library.
For pool exhaustion, log pool metrics (HikariCP exposes active, idle and pending counts). Exhaustion means connections are not being returned, which is a try-with-resources problem.
Enable -verbose:class temporarily to see which jar a class is loaded from when two versions are on the classpath.
Capture a heap dump on OOM with -XX:+HeapDumpOnOutOfMemoryError and analyse it rather than raising -Xmx blindly.

Tools worth reaching for

mvn dependency:tree
jcmd / jstack / jmap
-verbose:class
Eclipse MAT
JFR (Java Flight Recorder)

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

openjdk.org

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