I have written a Java command line tool to unpack ".pack.xz" archives…

…. It is based on the code of the Forge Installer.

Pack.xzExtractor/.classpath

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+<?xml version="1.0" encoding="UTF-8"?>
+<classpath>
+	<classpathentry kind="src" path="src"/>
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-1.7"/>
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER"/>
+	<classpathentry kind="lib" path="lib/xz-1.5.jar"/>
+	<classpathentry kind="output" path="bin"/>
+</classpath>

Pack.xzExtractor/.externalToolBuilders/Jar_Builder.launch

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+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<launchConfiguration type="org.eclipse.ant.AntBuilderLaunchConfigurationType">
+<booleanAttribute key="org.eclipse.ant.ui.ATTR_TARGETS_UPDATED" value="true"/>
+<booleanAttribute key="org.eclipse.ant.ui.DEFAULT_VM_INSTALL" value="false"/>
+<booleanAttribute key="org.eclipse.debug.ui.ATTR_LAUNCH_IN_BACKGROUND" value="false"/>
+<stringAttribute key="org.eclipse.jdt.launching.CLASSPATH_PROVIDER" value="org.eclipse.ant.ui.AntClasspathProvider"/>
+<stringAttribute key="org.eclipse.jdt.launching.PROJECT_ATTR" value=""/>
+<stringAttribute key="org.eclipse.ui.externaltools.ATTR_LOCATION" value="${workspace_loc:/Pack.xzExtractor/bin/build.xml}"/>
+<stringAttribute key="org.eclipse.ui.externaltools.ATTR_RUN_BUILD_KINDS" value="full,incremental,"/>
+<booleanAttribute key="org.eclipse.ui.externaltools.ATTR_TRIGGERS_CONFIGURED" value="true"/>
+</launchConfiguration>

Pack.xzExtractor/.project

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+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>Pack.xzExtractor</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+		<buildCommand>
+			<name>org.eclipse.ui.externaltools.ExternalToolBuilder</name>
+			<triggers>full,incremental,</triggers>
+			<arguments>
+				<dictionary>
+					<key>LaunchConfigHandle</key>
+					<value>&lt;project&gt;/.externalToolBuilders/Jar_Builder.launch</value>
+				</dictionary>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+	</natures>
+</projectDescription>

Pack.xzExtractor/.settings/org.eclipse.jdt.core.prefs

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+eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.7
+org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
+org.eclipse.jdt.core.compiler.compliance=1.7
+org.eclipse.jdt.core.compiler.debug.lineNumber=generate
+org.eclipse.jdt.core.compiler.debug.localVariable=generate
+org.eclipse.jdt.core.compiler.debug.sourceFile=generate
+org.eclipse.jdt.core.compiler.problem.assertIdentifier=error
+org.eclipse.jdt.core.compiler.problem.enumIdentifier=error
+org.eclipse.jdt.core.compiler.source=1.7

Pack.xzExtractor/.settings/org.eclipse.ltk.core.refactoring.prefs

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+eclipse.preferences.version=1
+org.eclipse.ltk.core.refactoring.enable.project.refactoring.history=false

Pack.xzExtractor/src/Main.java

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+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Arrays;
+import java.util.jar.JarEntry;
+import java.util.jar.JarOutputStream;
+import java.util.jar.Pack200;
+import org.tukaani.xz.XZInputStream;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+import java.nio.file.Path;
+
+public class Main {
+
+	public static void main(String[] args) throws IOException {
+		if (args.length > 2) {
+			System.out.println("Too many Arguments");
+		} else if (args.length < 2) {
+			System.out.println("Too little Arguments");
+		} else if (!new File(args[0]).exists()) {
+			System.out.println("InputFile does not exist");
+		} else {
+			try {
+				File input = new File(args[0]);
+				File output = new File(args[1]);
+				Path path = Paths.get(input.getAbsolutePath());
+				byte[] data = Files.readAllBytes(path);
+				unpackLibrary(output, data);
+				//success
+				System.exit(0);
+			} catch (Exception e) {
+				System.out.println("Error unpacking File: " + e.getLocalizedMessage());
+				//error
+				System.exit(1);
+			}
+		}
+	}
+
+	public static void unpackLibrary(File output, byte[] data)
+			throws IOException {
+		if (output.exists()) {
+			output.delete();
+		}
+
+		byte[] decompressed = readFully(new XZInputStream(
+				new ByteArrayInputStream(data)));
+
+		// Snag the checksum signature
+		String end = new String(decompressed, decompressed.length - 4, 4);
+		if (!end.equals("SIGN")) {
+			System.out.println("Unpacking failed, signature missing " + end);
+			return;
+		}
+
+		int x = decompressed.length;
+		int len = ((decompressed[x - 8] & 0xFF))
+				| ((decompressed[x - 7] & 0xFF) << 8)
+				| ((decompressed[x - 6] & 0xFF) << 16)
+				| ((decompressed[x - 5] & 0xFF) << 24);
+		byte[] checksums = Arrays.copyOfRange(decompressed, decompressed.length
+				- len - 8, decompressed.length - 8);
+
+		FileOutputStream jarBytes = new FileOutputStream(output);
+		JarOutputStream jos = new JarOutputStream(jarBytes);
+		Pack200.newUnpacker().unpack(new ByteArrayInputStream(decompressed),
+				jos);
+
+		jos.putNextEntry(new JarEntry("checksums.sha1"));
+		jos.write(checksums);
+		jos.closeEntry();
+
+		jos.close();
+		jarBytes.close();
+	}
+
+	public static byte[] readFully(InputStream stream) throws IOException {
+		byte[] data = new byte[4096];
+		ByteArrayOutputStream entryBuffer = new ByteArrayOutputStream();
+		int len;
+		do {
+			len = stream.read(data);
+			if (len > 0) {
+				entryBuffer.write(data, 0, len);
+			}
+		} while (len != -1);
+
+		return entryBuffer.toByteArray();
+	}
+
+}

Pack.xzExtractor/src/build.xml

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+<?xml version="1.0" ?>
+<!-- Configuration of the Ant build system to generate a Jar file -->
+<project name="Pack.xz_Extractor" default="CreateJar">
+    <property name="lib.dir"       value="../lib"/>
+	<target name="CreateJar" description="Create Jar file">
+		<jar jarfile="Pack_xz_Extractor.jar" basedir=".">
+			<manifest>
+				<attribute name="Main-Class" value="Main" />
+			</manifest>
+	        <zipgroupfileset dir="${lib.dir}" includes="**/*.jar" />
+			<fileset dir="../bin" includes="**/*.class" />
+		</jar>
+	</target>
+</project>