您可以使用Pack200文件而無需首先解壓縮它？

Question

是否有可能從Pack200創建的pack.gz文件中即時加載類（在內存中），而無需首先將其解壓縮回jar中？我可以找到的所有示例都向我展示瞭如何將其解壓縮爲.jar文件，然後從.jar文件加載類。

Answer 1

是的，這是可能的。 Pack200.Unpacker.unpack方法寫入JarOutputStream;如果您在後臺線程中這樣做，則可以使用管道將新的JarOutputStream連接到JarInputStream，然後從中讀取。

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); 
} 

這應該足夠了。但是，仍然存在兩個問題：

• 如果解包操作出現錯誤，您將永遠不會知道它，因爲ExecutorServices會抑制它們的任務'異常。
• JarInputStream和JarOutputStream（更具體地說，它們的超類，InflaterInputStream和DeflaterOutputStream）不適用於管道。這是因爲關閉JarOutputStream強制調用其finish()方法，但管道另一側的JarInputStream將永遠不會讀取該數據。這意味着在調用finish()之前幾乎肯定會關閉JarInputStream，導致finish()由於管道損壞而生成IOException。

要解決第一個問題，我們可以重寫JarInputStream的close方法來解決解包操作中的任何故障。要解決第二，我們可以在覆蓋的JarOutputStream到finish()什麼都不做：

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; 
      } 
     } 
    }; 
}