创建 ClassLoader 以从字节数组加载 JAR 文件 [英] Creating a ClassLoader to load a JAR file from a byte array

问题描述

我希望编写一个自定义类加载器，它将通过自定义网络加载一个 JAR 文件.最后，我需要处理的只是 JAR 文件的字节数组.

I'm looking to write a custom class loader that will load a JAR file from across a custom network. In the end, all I have to work with is a byte array of the JAR file.

我无法将字节数组转储到文件系统并使用 URLClassLoader.
我的第一个计划是从流或字节数组创建一个 JarFile 对象，但它只支持一个 File 对象.

I cannot dump the byte array onto the file system and use a URLClassLoader.
My first plan was to create a JarFile object from a stream or byte array, but it only supports a File object.

我已经写了一些使用 JarInputStream 的东西:

I've already written up something that uses a JarInputStream:

public class RemoteClassLoader extends ClassLoader {

    private final byte[] jarBytes;

    public RemoteClassLoader(byte[] jarBytes) {
        this.jarBytes = jarBytes;
    }

    @Override
    public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        Class<?> clazz = findLoadedClass(name);
        if (clazz == null) {
            try {
                InputStream in = getResourceAsStream(name.replace('.', '/') + ".class");
                ByteArrayOutputStream out = new ByteArrayOutputStream();
                StreamUtils.writeTo(in, out);
                byte[] bytes = out.toByteArray();
                clazz = defineClass(name, bytes, 0, bytes.length);
                if (resolve) {
                    resolveClass(clazz);
                }
            } catch (Exception e) {
                clazz = super.loadClass(name, resolve);
            }
        }
        return clazz;
    }

    @Override
    public URL getResource(String name) {
        return null;
    }

    @Override
    public InputStream getResourceAsStream(String name) {
        try (JarInputStream jis = new JarInputStream(new ByteArrayInputStream(jarBytes))) {
            JarEntry entry;
            while ((entry = jis.getNextJarEntry()) != null) {
                if (entry.getName().equals(name)) {
                    return jis;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }
}

这对于小的 JAR 文件可能工作正常，但我尝试加载一个 2.7MB jar 文件，其中包含几乎 2000 个类，结果是大约需要 160 毫秒 来遍历所有条目，更不用说加载它找到的类了.

This may work fine for small JAR files, but I tried loading up a 2.7MB jar file with almost 2000 classes and it was taking around 160 ms just to iterate through all the entries let alone load the class it found.

如果有人知道比每次加载类时遍历 JarInputStream 的条目更快的解决方案，请分享！

If anyone knows a solution that's faster than iterating through a JarInputStream's entries each time a class is loaded, please share!

推荐答案

我会遍历类一次并缓存条目.我还会查看 URLClassLoader 的源代码，看看它是如何做到的.如果失败，请将数据写入临时文件并通过普通类加载器加载.

I would iterate through the class once and cache the entries. I would also look at the source code for URLClassLoader to see how it does it. If that fails, write the data to a temporary file and load it via the normal class loader.

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