您可以使用Pack200文件而无需首先解压缩它？

Question

是否有可能从Pack200创建的pack.gz文件中即时加载类（在内存中），而无需首先将其解压缩回jar中？我可以找到的所有示例都向我展示了如何将其解压缩为.jar文件，然后从.jar文件加载类。

Answer 1

是的，这是可能的。 Pack200.Unpacker.unpack方法写入JarOutputStream;如果您在后台线程中这样做，则可以使用管道将新的JarOutputStream连接到JarInputStream，然后从中读取。

public JarInputStream readPackFile(Path packGzFile) 
throws IOException { 

    PipedInputStream pipeIn = new PipedInputStream(); 
    PipedOutputStream pipeOut = new PipedOutputStream(pipeIn); 

    ExecutorService executor = Executors.newSingleThreadExecutor(); 
    Callable<Void> unpacker = new Callable<Void>() { 
     @Override 
     public Void call() 
     throws IOException { 
      try (InputStream file = 
        new GZIPInputStream(
         new BufferedInputStream(
          Files.newInputStream(packGzFile))); 
       JarOutputStream jarOutput = new JarOutputStream(pipeOut)) { 

       Pack200.newUnpacker().unpack(file, jarOutput); 
       return null; 
      } finally { 
       executor.shutdown(); 
      } 
     } 
    }; 
    executor.submit(unpacker); 

    return new JarInputStream(pipeIn); 
} 

这应该足够了。但是，仍然存在两个问题：

• 如果解包操作出现错误，您将永远不会知道它，因为ExecutorServices会抑制它们的任务'异常。
• JarInputStream和JarOutputStream（更具体地说，它们的超类，InflaterInputStream和DeflaterOutputStream）不适用于管道。这是因为关闭JarOutputStream强制调用其finish()方法，但管道另一侧的JarInputStream将永远不会读取该数据。这意味着在调用finish()之前几乎肯定会关闭JarInputStream，导致finish()由于管道损坏而生成IOException。

要解决第一个问题，我们可以重写JarInputStream的close方法来解决解包操作中的任何故障。要解决第二，我们可以在覆盖的JarOutputStream到finish()什么都不做：

public JarInputStream readPackFile(Path packGzFile) 
throws IOException { 

    PipedInputStream pipeIn = new PipedInputStream(); 
    PipedOutputStream pipeOut = new PipedOutputStream(pipeIn); 

    class NonFinishingJarOutputStream 
    extends JarOutputStream { 
     NonFinishingJarOutputStream(OutputStream out) 
     throws IOException { 
      super(out); 
     } 

     @Override 
     public void finish() 
     throws IOException { 
      // Deliberately empty. 
     } 
    } 

    ExecutorService executor = Executors.newSingleThreadExecutor(); 
    Callable<Void> unpacker = new Callable<Void>() { 
     @Override 
     public Void call() 
     throws IOException { 
      try (InputStream file = 
        new GZIPInputStream(
         new BufferedInputStream(
          Files.newInputStream(packGzFile))); 
       JarOutputStream jarOutput = 
        new NonFinishingJarOutputStream(pipeOut)) { 

       Pack200.newUnpacker().unpack(file, jarOutput); 
       return null; 
      } finally { 
       executor.shutdown(); 
      } 
     } 
    }; 
    Future<?> unpackerTask = executor.submit(unpacker); 

    return new JarInputStream(pipeIn) { 
     @Override 
     public void close() 
     throws IOException { 
      super.close(); 
      try { 
       // If the unpack generated an exception, propagate it here. 
       unpackerTask.get(); 
      } catch (ExecutionException e) { 
       throw new IOException(e); 
      } catch (InterruptedException e) { 
       InterruptedIOException iie = new InterruptedIOException(); 
       iie.initCause(e); 
       throw iie; 
      } 
     } 
    }; 
}