wu-sheng commented on a change in pull request #4858:
URL: https://github.com/apache/skywalking/pull/4858#discussion_r447485936



##########
File path: 
apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/plugin/bytebuddy/CacheableTransformerDecorator.java
##########
@@ -0,0 +1,194 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package org.apache.skywalking.apm.agent.core.plugin.bytebuddy;
+
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.agent.builder.ResettableClassFileTransformer;
+import net.bytebuddy.utility.RandomString;
+import org.apache.skywalking.apm.agent.core.boot.AgentPackageNotFoundException;
+import org.apache.skywalking.apm.agent.core.boot.AgentPackagePath;
+import org.apache.skywalking.apm.agent.core.logging.api.ILog;
+import org.apache.skywalking.apm.agent.core.logging.api.LogManager;
+import org.apache.skywalking.apm.agent.core.util.FileUtils;
+import org.apache.skywalking.apm.agent.core.util.IOUtils;
+
+import java.io.ByteArrayInputStream;
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.lang.instrument.IllegalClassFormatException;
+import java.security.ProtectionDomain;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * Wrapper classFileTransformer of ByteBuddy, save the enhanced bytecode to 
memory cache or file cache,
+ * and automatically load the previously generated bytecode during the second 
retransform,
+ * to solve the problem that ByteBuddy generates auxiliary classes with 
different random names every time.
+ * Allow other javaagent to enhance those classes that enhanced by SkyWalking 
agent.
+ */
+public class CacheableTransformerDecorator implements 
AgentBuilder.TransformerDecorator {
+
+    private static final ILog logger = 
LogManager.getLogger(CacheableTransformerDecorator.class);
+
+    private final ClassCacheMode cacheMode;
+    private ClassCacheResolver cacheResolver;
+
+    public CacheableTransformerDecorator(ClassCacheMode cacheMode) throws 
IOException {
+        this.cacheMode = cacheMode;
+        initClassCache();
+    }
+
+    private void initClassCache() throws IOException {
+        if (this.cacheMode.equals(ClassCacheMode.FILE)) {
+            String cacheDirBase = null;
+            try {
+                cacheDirBase = AgentPackagePath.getPath() + "/class-cache";
+            } catch (AgentPackageNotFoundException e) {
+                throw new IOException("Can't find the root path for creating 
/class-cache folder.");
+            }
+            File cacheDir = new File(cacheDirBase + "/class-cache-" + 
RandomString.make());
+            if (!cacheDir.exists()) {
+                cacheDir.mkdirs();
+            }
+            if (!cacheDir.exists()) {
+                throw new IOException("Create class cache dir failure");
+            }
+
+            cacheResolver = new FileCacheResolver(cacheDir);
+        } else {
+            cacheResolver = new MemoryCacheResolver();
+        }
+    }
+
+    @Override
+    public ResettableClassFileTransformer 
decorate(ResettableClassFileTransformer classFileTransformer) {
+        return new 
ResettableClassFileTransformer.WithDelegation(classFileTransformer) {
+
+            @Override
+            public byte[] transform(ClassLoader loader, String className, 
Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] 
classfileBuffer) throws IllegalClassFormatException {
+                // load from cache
+                byte[] classCache = cacheResolver.getClassCache(loader, 
className);
+                if (classCache != null) {
+                    return classCache;
+                }
+
+                //transform class
+                classfileBuffer = classFileTransformer.transform(loader, 
className, classBeingRedefined, protectionDomain, classfileBuffer);
+
+                // save to cache
+                if (classfileBuffer != null) {
+                    cacheResolver.putClassCache(loader, className, 
classfileBuffer);
+                }
+
+                return classfileBuffer;
+            }
+        };
+    }
+
+    private static String getClassLoaderHash(ClassLoader loader) {
+        String classloader;
+        if (loader != null) {
+            classloader = Integer.toHexString(loader.hashCode());
+        } else {
+            //classloader is null for BootstrapClassLoader
+            classloader = "00000000";
+        }
+        return classloader;
+    }
+
+    interface ClassCacheResolver {
+
+        byte[] getClassCache(ClassLoader loader, String className);
+
+        void putClassCache(ClassLoader loader, String className, byte[] 
classfileBuffer);
+    }
+
+    static class MemoryCacheResolver implements ClassCacheResolver {
+        // classloaderHashcode@className -> class bytes
+        private Map<String, byte[]> classCacheMap = new 
ConcurrentHashMap<String, byte[]>();
+
+        @Override
+        public byte[] getClassCache(ClassLoader loader, String className) {
+            String cacheKey = getCacheKey(loader, className);
+            return classCacheMap.get(cacheKey);
+        }
+
+        @Override
+        public void putClassCache(ClassLoader loader, String className, byte[] 
classfileBuffer) {
+            String cacheKey = getCacheKey(loader, className);
+            classCacheMap.put(cacheKey, classfileBuffer);
+        }
+
+        private String getCacheKey(ClassLoader loader, String className) {
+            return getClassLoaderHash(loader) + "@" + className;
+        }
+    }
+
+    static class FileCacheResolver implements ClassCacheResolver {
+
+        private final File cacheDir;
+
+        FileCacheResolver(File cacheDir) {
+            this.cacheDir = cacheDir;
+
+            //clean cache dir on exit
+            FileUtils.deleteDirectoryOnExit(cacheDir);
+        }
+
+        @Override
+        public byte[] getClassCache(ClassLoader loader, String className) {
+            // load from cache
+            File cacheFile = getCacheFile(loader, className);
+            if (cacheFile.exists()) {
+                FileInputStream fileInputStream = null;
+                try {
+                    fileInputStream = new FileInputStream(cacheFile);
+                    return IOUtils.toByteArray(fileInputStream);
+                } catch (IOException e) {
+                    logger.error("load class bytes from cache file failure", 
e);
+                } finally {
+                    IOUtils.closeQuietly(fileInputStream);
+                }
+            }
+            return null;
+        }
+
+        @Override
+        public void putClassCache(ClassLoader loader, String className, byte[] 
classfileBuffer) {
+            File cacheFile = getCacheFile(loader, className);
+            FileOutputStream output = null;
+            try {
+                output = new FileOutputStream(cacheFile);
+                IOUtils.copy(new ByteArrayInputStream(classfileBuffer), 
output);
+            } catch (IOException e) {
+                logger.error("save class bytes to cache file failure", e);
+            } finally {
+                IOUtils.closeQuietly(output);
+            }
+        }
+
+        private File getCacheFile(ClassLoader loader, String className) {
+            String filename = getClassLoaderHash(loader) + "@" + 
className.replace('/', '.') + ".class";

Review comment:
       You are putting all files in one folder, NOTICE potential issue
   
   FAT32: Maximum number of files per directory: 216 - 1 (65,535)
   NTFS:Maximum number of files: 232 - 1 (4,294,967,295)
   ext2/ext3:Maximum number of files per directory: ~1.3 × 1020 (performance 
issues past 10,000)
   ext4: Maximum number of files per directory: unlimited
   
   Material source, 
https://stackoverflow.com/questions/466521/how-many-files-can-i-put-in-a-directory




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