Java：在运行时将类添加到 Jar 存档

Question

我想在运行时将一些已编译的类（.class 文件）添加到当前 Jar 文件中的目录（包）
我怎样才能做到这一点？

谢谢

Answer 1

这是无法做到的 - 要更新 Jar 文件，您需要创建一个新文件并用新文件覆盖旧文件。

以下是有关如何执行此操作的示例：

import java.io.*;
import java.util.*;
import java.util.zip.*;
import java.util.jar.*;

public class JarUpdate {
   /**
    * main()
    */
   public static void main(String[] args) throws IOException {
      // Get the jar name and entry name from the command-line.

      String jarName = args[0];
      String fileName = args[1];

      // Create file descriptors for the jar and a temp jar.

      File jarFile = new File(jarName);
      File tempJarFile = new File(jarName + ".tmp");

      // Open the jar file.

      JarFile jar = new JarFile(jarFile);
      System.out.println(jarName + " opened.");

      // Initialize a flag that will indicate that the jar was updated.

      boolean jarUpdated = false;

      try {
         // Create a temp jar file with no manifest. (The manifest will
         // be copied when the entries are copied.)

         Manifest jarManifest = jar.getManifest();
         JarOutputStream tempJar =
            new JarOutputStream(new FileOutputStream(tempJarFile));

         // Allocate a buffer for reading entry data.

         byte[] buffer = new byte[1024];
         int bytesRead;

         try {
            // Open the given file.

            FileInputStream file = new FileInputStream(fileName);

            try {
               // Create a jar entry and add it to the temp jar.

               JarEntry entry = new JarEntry(fileName);
               tempJar.putNextEntry(entry);

               // Read the file and write it to the jar.

               while ((bytesRead = file.read(buffer)) != -1) {
                  tempJar.write(buffer, 0, bytesRead);
               }

               System.out.println(entry.getName() + " added.");
            }
            finally {
               file.close();
            }

            // Loop through the jar entries and add them to the temp jar,
            // skipping the entry that was added to the temp jar already.

            for (Enumeration entries = jar.entries(); entries.hasMoreElements(); ) {
               // Get the next entry.

               JarEntry entry = (JarEntry) entries.nextElement();

               // If the entry has not been added already, add it.

               if (! entry.getName().equals(fileName)) {
                  // Get an input stream for the entry.

                  InputStream entryStream = jar.getInputStream(entry);

                  // Read the entry and write it to the temp jar.

                  tempJar.putNextEntry(entry);

                  while ((bytesRead = entryStream.read(buffer)) != -1) {
                     tempJar.write(buffer, 0, bytesRead);
                  }
               }
            }

            jarUpdated = true;
         }
         catch (Exception ex) {
            System.out.println(ex);

            // Add a stub entry here, so that the jar will close without an
            // exception.

            tempJar.putNextEntry(new JarEntry("stub"));
         }
         finally {
            tempJar.close();
         }
      }
      finally {
         jar.close();
         System.out.println(jarName + " closed.");

         // If the jar was not updated, delete the temp jar file.

         if (! jarUpdated) {
            tempJarFile.delete();
         }
      }

      // If the jar was updated, delete the original jar file and rename the
      // temp jar file to the original name.

      if (jarUpdated) {
         jarFile.delete();
         tempJarFile.renameTo(jarFile);
         System.out.println(jarName + " updated.");
      }
   }
}

Answer 2

我不太确定，但我认为不可能从 JAR 文件（我们称之为 foo.jar）加载类，然后修改加载类的同一个 JAR 文件，添加一个新的类并期望类加载器找到该类。

我会考虑重构应用程序本身并使其能够动态加载类（使用 URLClassLoader 或任何其他技术），而不是尝试强制执行您描述的单个 JAR 行为。