javac -Xlint: show all compiler warnings (unchecked, deprecation, rawtypes)

javac -Xlint Main.java enables all recommended compiler warnings and prints each one with its location, including the ones javac otherwise only summarises as Note: Main.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details. You can also pick categories: -Xlint:unchecked,deprecation, or everything except some: -Xlint:all,-serial. Combine with -Werror to make warnings fail the build.

Default versus -Xlint versus -Werror

cat > Lint.java <<'EOF'
import java.util.ArrayList;
import java.util.List;

public class Lint {
    static List raw() { return new ArrayList(); }

    public static void main(String[] args) {
        List<String> names = raw();
        Integer boxed = new Integer(5);
        switch (args.length) {
            case 0: System.out.println("none");
            case 1: System.out.println("one");
        }
        System.out.println(names.size() + boxed);
    }
}
EOF
echo "== default"
javac -d out Lint.java; echo "exit: $?"
echo "== -Xlint:unchecked,fallthrough"
javac -Xlint:unchecked,fallthrough -d out Lint.java 2>&1 | grep 'warning:'; echo "exit: ${PIPESTATUS[0]}"
echo "== -Xlint"
javac -Xlint -d out Lint.java 2>&1 | grep -E 'warning:|warnings$'
echo "== -Xlint:all,-rawtypes -Werror"
javac -Xlint:all,-rawtypes -Werror -d out Lint.java 2>&1 | grep -E 'warning:|error'; echo "exit: ${PIPESTATUS[0]}"

== default
Lint.java:9: warning: [removal] Integer(int) in Integer has been deprecated and marked for removal
        Integer boxed = new Integer(5);
                        ^
Note: Lint.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 warning
exit: 0
== -Xlint:unchecked,fallthrough
Lint.java:8: warning: [unchecked] unchecked conversion
Lint.java:9: warning: [removal] Integer(int) in Integer has been deprecated and marked for removal
Lint.java:12: warning: [fallthrough] possible fall-through into case
exit: 0
== -Xlint
Lint.java:5: warning: [rawtypes] found raw type: List
Lint.java:5: warning: [rawtypes] found raw type: ArrayList
Lint.java:8: warning: [unchecked] unchecked conversion
Lint.java:9: warning: [removal] Integer(int) in Integer has been deprecated and marked for removal
Lint.java:12: warning: [fallthrough] possible fall-through into case
5 warnings
== -Xlint:all,-rawtypes -Werror
Lint.java:8: warning: [unchecked] unchecked conversion
Lint.java:9: warning: [removal] Integer(int) in Integer has been deprecated and marked for removal
error: warnings found and -Werror specified
1 error
exit: 1

By default only the [removal] warning appears, and the unchecked conversion is hidden behind a Note. -Xlint lists the raw types, the unchecked conversion, the removal and the switch fall-through. -Werror turns the run into a failure (exit 1) with error: warnings found and -Werror specified. Disabling rawtypes with -rawtypes removes those two warnings, and in this run javac stopped before it reported the fall-through, so fix warnings in the order they appear and re-run.

Frequent keys

javac --help-lint prints the keys your JDK supports. A minus sign disables a key (-Xlint:-options), and keys combine with commas.

In build tools

<configuration>
  <compilerArgs>
    <arg>-Xlint:all,-serial</arg>
    <arg>-Werror</arg>
  </compilerArgs>
</configuration>

tasks.withType<JavaCompile>().configureEach {
    options.compilerArgs.addAll(listOf("-Xlint:all,-serial", "-Werror"))
}

Pitfalls

• Warnings are not errors: -Xlint never finds problems such as cannot find symbol, which already stop the build.
• -Werror with -Xlint:all can break the build on a JDK upgrade, because newer JDKs add categories and deprecate APIs. Pin the JDK in CI, or exclude noisy categories explicitly.
• @SuppressWarnings takes the same key names as -Xlint (unchecked, rawtypes, deprecation...). IDEs and static analysers add names of their own, which javac ignores.
• When migrating to a newer target with javac --release, -Xlint:deprecation,removal shows which APIs to replace before they disappear.

FAQ