What causes "JNA temporary directory 'jnatmp' is not writable" in java-native-access/jna?

chown/chmod the temp directory so the JVM user can write to it (e.g. chmod 1777 like /tmp).

java-native-access/jna · critical · java.io.IOException

JNA temporary directory 'jnatmp' is not writable

Error message

JNA temporary directory 'jnatmp' is not writable

What it means

JNA stages its native library into a temporary directory and requires write access there to extract the .so/.dll/.jnilib file. After confirming the directory exists, it checks canWrite(); if the process lacks write permission it throws this IOException. Without write access JNA cannot materialize the native library, so native calls cannot proceed.

Source

Thrown at src/com/sun/jna/Native.java:1425

                }
                jnatmp = new File(xdgCacheFile, "JNA/temp");
            } else {
                // Loading DLLs via System.load() under a directory with a unicode
                // name will fail on windows, so use a hash code of the user's
                // name in case the user's name contains non-ASCII characters
                jnatmp = new File(tmp, "jna-" + System.getProperty("user.name").hashCode());
            }

            jnatmp.mkdirs();
            if (!jnatmp.exists() || !jnatmp.canWrite()) {
                jnatmp = tmp;
            }
        }
        if (!jnatmp.exists()) {
            throw new IOException("JNA temporary directory '" + jnatmp + "' does not exist");
        }
        if (!jnatmp.canWrite()) {
            throw new IOException("JNA temporary directory '" + jnatmp + "' is not writable");
        }
        return jnatmp;
    }

    /** Remove all marked temporary files in the given directory. */
    static void removeTemporaryFiles() throws IOException {
        File dir = getTempDir();
        FilenameFilter filter = new FilenameFilter() {
            @Override
            public boolean accept(File dir, String name) {
                return name.endsWith(".x") && name.startsWith(JNA_TMPLIB_PREFIX);
            }
        };
        File[] files = dir.listFiles(filter);
        for (int i=0;files != null && i < files.length;i++) {
            File marker = files[i];
            String name = marker.getName();
            name = name.substring(0, name.length()-2);

View on GitHub (pinned to d036ad9781)

Solutions

chown/chmod the temp directory so the JVM user can write to it (e.g. chmod 1777 like /tmp).
Point JNA at a writable directory with -Djna.tmpdir=/path/writable (or JNA_TMPDIR env var).
Run the process under a user that owns or can write to java.io.tmpdir.
Prefer bundling the native library on java.library.path or use OSGi-native bundling so JNA does not need to extract to a temp dir.

Example fix

// before
ls -ld /opt/jnatmp  # drwxr-xr-x root root; app runs as 'appuser' -> not writable

// after
sudo chown appuser /opt/jnatmp
java -Djna.tmpdir=/opt/jnatmp -jar app.jar

Defensive patterns

Strategy: validation

Validate before calling

File dir = new File(System.getProperty("jna.tmpdir",
    System.getProperty("java.io.tmpdir")));
if (dir.isDirectory() && !dir.canWrite()) {
    throw new IllegalStateException("jna.tmpdir must be writable by user " + System.getProperty("user.name"));
}

Try / catch

try {
    MyLib lib = Native.load("mylib", MyLib.class);
} catch (IOException e) {
    if (e.getMessage().contains("is not writable")) {
        System.setProperty("jna.tmpdir", "/var/tmp/jna");
        // retry load
    } else { throw e; }
}

Prevention

Run the JVM under a user that owns or can write to the temp dir; mirror /tmp permissions (1777).
Mount a writable volume and point -Djna.tmpdir at it in hardened containers.
Check canWrite() on the configured dir during application startup, before first native call.
Avoid shared temp dirs created by other users with restrictive umasks.

When it happens

Trigger: First JNA library load when the resolved temp directory (jna.tmpdir or jnatmp* under java.io.tmpdir) exists but is not writable by the JVM's OS user.

Common situations: Running a JVM as a non-root user while the temp dir is root-owned, hardened containers with read-only volumes, or a shared jna.tmpdir created earlier by another user with restrictive permissions.

Understand the failure class

Background: "Permission denied" / "Failed to write" file errors: why a library can't write its files to disk (EACCES, EPERM, ENOSPC) and how to fix them — this error's family across 43 libraries.

Related errors

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/13e98f8f5cb2b351. Report an issue: GitHub.