diff --git a/util/build.gradle.kts b/util/build.gradle.kts
index d0a49c4b..ec45e637 100644
--- a/util/build.gradle.kts
+++ b/util/build.gradle.kts
@@ -3,6 +3,10 @@ plugins {
     kotlin("jvm")
 }
 
+dependencies {
+    testImplementation("com.google.jimfs:jimfs:${Versions.jimfs}")
+}
+
 publishing {
     publications.create<MavenPublication>("maven") {
         from(components["java"])
diff --git a/util/src/test/java/dev/openrs2/util/io/DeterministicJarOutputStreamTest.kt b/util/src/test/java/dev/openrs2/util/io/DeterministicJarOutputStreamTest.kt
new file mode 100644
index 00000000..0724e2f3
--- /dev/null
+++ b/util/src/test/java/dev/openrs2/util/io/DeterministicJarOutputStreamTest.kt
@@ -0,0 +1,107 @@
+package dev.openrs2.util.io
+
+import com.google.common.jimfs.Configuration
+import com.google.common.jimfs.Jimfs
+import java.io.ByteArrayInputStream
+import java.io.ByteArrayOutputStream
+import java.nio.file.Files
+import java.nio.file.attribute.FileTime
+import java.util.jar.Attributes
+import java.util.jar.JarEntry
+import java.util.jar.JarInputStream
+import java.util.jar.JarOutputStream
+import java.util.jar.Manifest
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotNull
+import kotlin.test.assertNull
+
+object DeterministicJarOutputStreamTest {
+    private val UNIX_EPOCH = FileTime.fromMillis(0)
+    private val Y2K = FileTime.fromMillis(946684800)
+    private val manifest = Manifest().apply {
+        mainAttributes[Attributes.Name.MANIFEST_VERSION] = "1.0"
+        mainAttributes[Attributes.Name.MAIN_CLASS] = "Hello"
+    }
+
+    @Test
+    fun testPutNextEntry() {
+        ByteArrayOutputStream().use { out ->
+            DeterministicJarOutputStream.create(out).use { jar ->
+                val entry = JarEntry("Hello.class")
+                entry.creationTime = Y2K
+                entry.lastAccessTime = Y2K
+                entry.lastModifiedTime = Y2K
+                jar.putNextEntry(entry)
+            }
+
+            JarInputStream(ByteArrayInputStream(out.toByteArray())).use { jar ->
+                assertNull(jar.manifest)
+
+                val entry = jar.nextJarEntry
+                assertNotNull(entry)
+
+                assertEquals("Hello.class", entry.name)
+                assertEquals(0, entry.time)
+                assertEquals(UNIX_EPOCH, entry.creationTime)
+                assertEquals(UNIX_EPOCH, entry.lastAccessTime)
+                assertEquals(UNIX_EPOCH, entry.lastModifiedTime)
+
+                assertNull(jar.nextJarEntry)
+            }
+        }
+    }
+
+    @Test
+    fun testManifest() {
+        ByteArrayOutputStream().use { out ->
+            DeterministicJarOutputStream.create(out, manifest).use { jar ->
+                jar.putNextEntry(JarEntry("Hello.class"))
+            }
+
+            JarInputStream(ByteArrayInputStream(out.toByteArray())).use { jar ->
+                assertEquals(manifest, jar.manifest)
+            }
+        }
+    }
+
+    @Test
+    fun testRepack() {
+        Jimfs.newFileSystem(Configuration.unix()).use { fs ->
+            val originalFile = Files.createTempFile(fs.getPath("/"), "original", ".jar")
+            try {
+                JarOutputStream(Files.newOutputStream(originalFile)).use { jar ->
+                    val entry = JarEntry("Hello.class")
+                    entry.creationTime = Y2K
+                    entry.lastAccessTime = Y2K
+                    entry.lastModifiedTime = Y2K
+                    jar.putNextEntry(entry)
+                }
+
+                val repackedFile = Files.createTempFile(fs.getPath("/"), "repacked", ".jar")
+                try {
+                    DeterministicJarOutputStream.repack(originalFile, repackedFile)
+
+                    JarInputStream(Files.newInputStream(repackedFile)).use { jar ->
+                        assertNull(jar.manifest)
+
+                        val entry = jar.nextJarEntry
+                        assertNotNull(entry)
+
+                        assertEquals("Hello.class", entry.name)
+                        assertEquals(0, entry.time)
+                        assertEquals(UNIX_EPOCH, entry.creationTime)
+                        assertEquals(UNIX_EPOCH, entry.lastAccessTime)
+                        assertEquals(UNIX_EPOCH, entry.lastModifiedTime)
+
+                        assertNull(jar.nextJarEntry)
+                    }
+                } finally {
+                    Files.deleteIfExists(repackedFile)
+                }
+            } finally {
+                Files.deleteIfExists(originalFile)
+            }
+        }
+    }
+}