共计 8086 个字符，预计需要花费 21 分钟才能阅读完成。
起因
在一次我的项目问题剖析中，须要获取以后内存中的所有 classloader，以及它们之间的父子关系。
借助于 Java agent 运行期动静 attach 到指标 jvm 的个性，以及通过 Instrumentation 对象来拿到所有加载的 class，于是有了上面的工具。
参考：一波三折！记一次非堆内存透露 (CXF+Jackson) 的排查
理论应用中发现的问题
在编写 agent 的过程中，还发现了另一个问题，即 agent.jar 文件由指标 jvm 的 SystemClassLoader 加载，导致 agent 代码批改后，
再次 attach 到指标 jvm 上时，理论加载的依然是第一次 attach 的 class 内容，即便前一个 agent.jar 文件曾经被删除也没用。
为了解决这个问题，最初通过一个独立的、每次用完就抛弃的 agent classloader 来实现每次 attach 时可执行不同的 agent 逻辑。
agent 入口代码
public class AgentClassLoader extends URLClassLoader {public AgentClassLoader(URL[] urls, ClassLoader parent) {super(urls, parent);
}
@Override
protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {synchronized (getClassLoadingLock(name)) {
//1.findLoadedClass
Class<?> c = findLoadedClass(name);
if (c == null) {
// 如果找不到，会丢出异样
if (name.startsWith("toone.agent.runner.")) {
//2. runner 从本地查找
c = findClass(name);
}
else {
//2. 其余走默认逻辑加载
c = super.loadClass(name, false);
}
}
//3. 解析
if (resolve) {resolveClass(c);
}
return c;
}
}
}
public class AgentMain {@SuppressWarnings("unchecked")
public static void agentmain(String agentArgument, Instrumentation instrumentation) throws Exception {
AgentClassLoader classLoader = null;
try {
//1. 参数解析，提取 logdir
Map<String, String> args = parseArgs(agentArgument);
String logdir = args.get("o");
logdir = URLDecoder.decode(logdir, "UTF-8");
//2. 筹备上下文参数
Map<String, Object> context = new HashMap<String, Object>();
context.put("instrumentation", instrumentation);
context.put("logdir", new File(logdir));
//3. 实例化 agentrunner
URL agentPath = AgentMain.class.getProtectionDomain().getCodeSource().getLocation();
// 作用是应用独立的 classloader 来加载 真正的 agent 逻辑局部，并且在每次执行完该 loader 都是可回收的，// 这样能够屡次 attach 到 jvm 上，执行不同的 agent 逻辑
classLoader = new AgentClassLoader(new URL[] {agentPath}, ClassLoader.getSystemClassLoader());
Class<?> agentRunnerClass = classLoader.loadClass("toone.agent.runner.AgentRunner");
Consumer<Map<String, Object>> agentRunner = (Consumer<Map<String, Object>>) agentRunnerClass.newInstance();
//4. 执行
agentRunner.accept(context);
}
catch (Throwable e) {
// 如果记录日志失败，那么输入到执行系统日志中
Logger.getLogger(AgentMain.class.getName()).log(Level.WARNING, "启动 agentmain 失败！", e);
}
finally {if (classLoader != null) {classLoader.close();
}
}
}
private static Map<String, String> parseArgs(String agentArgument) {Map<String, String> args = new HashMap<String, String>();
if (agentArgument != null) {String[] opts = agentArgument.split(",");
for (String opt : opts) {String[] parts = opt.split("=");
String key = parts[0].trim();
String value = (parts.length <= 1) ? "" : parts[1].trim();
args.put(key, value);
}
}
return args;
}
}agent runner 入口
public class AgentRunner implements Consumer<Map<String, Object>> {
@Override
public void accept(Map<String, Object> context) {Instrumentation instrumentation = (Instrumentation) context.get("instrumentation");
File logdir = (File) context.get("logdir");
long now = System.currentTimeMillis();
String today = new SimpleDateFormat("yyyy-MM-dd").format(now);
String time = new SimpleDateFormat("HHmmss").format(now);
String fileNamePrefix = today + ".T" + time;
StringBuilder buf = new StringBuilder();
try {
//1.GC 以前内存和 class 数量
logMemoryUsage(buf, ManagementFactory.getMemoryMXBean().getNonHeapMemoryUsage());
logClassesInfo(buf, ManagementFactory.getClassLoadingMXBean());
//2. GC
buf.append("\n 开始 GC...\n");
System.gc();
Thread.sleep(1000);
//3.GC 当前的内存和 class 数量
logMemoryUsage(buf, ManagementFactory.getMemoryMXBean().getNonHeapMemoryUsage());
logClassesInfo(buf, ManagementFactory.getClassLoadingMXBean());
//6. 记录 class loader 的信息
new ClassLoaderAnalyser().record(logdir, fileNamePrefix, instrumentation);
}
catch (Throwable e) {logError(buf, "agent 执行失败", e);
}
finally {writeLogFile(buf, logdir, fileNamePrefix);
}
}
}classloader 分析器
public class ClassLoaderAnalyser {public void record(File logdir, String fileNamePrefix, Instrumentation instrumentation) {Map<ClassLoader, ClassLoaderWraper> loaders = new HashMap<ClassLoader, ClassLoaderWraper>();
ClassLoaderWraper root = new ClassLoaderWraper(null);
try {Class<?>[] loaded = instrumentation.getAllLoadedClasses();
for (Class<?> clazz : loaded) {if (clazz == null) {continue;}
ClassLoader loader = clazz.getClassLoader();
// 将以后 loader 增加到 parent.children 中
ClassLoaderWraper wraper = addToLoaderTree(root, loaders, loader);
wraper.count++;
}
StringBuilder buf = new StringBuilder();
// 以 yml 格局记录树信息
traversalLoaderTree(buf, root);
writeLogFile(buf, logdir, fileNamePrefix);
}
catch (Throwable e) {
// 如果记录日志失败，那么输入到执行系统日志中
Logger.getLogger(ClassLoaderAnalyser.class.getName()).log(Level.WARNING, "记录 class loader 信息失败！", e);
}
}
/**
* 将给定 loader 增加到 parent.children 中
*
* @param loaders 全副已知 loader 的汇合，在这外面寻找父节点
* @param loader 须要增加到 loader tree 上的节点
* @return loader 的父节点
*/
private ClassLoaderWraper addToLoaderTree(ClassLoaderWraper root, Map<ClassLoader, ClassLoaderWraper> loaders,
ClassLoader loader) {if (loader == null) {return root;}
ClassLoaderWraper self = loaders.get(loader);
if (self != null) {
// 以后 loader 曾经在树中，就间接返回
return self;
}
// 找到 parent
ClassLoaderWraper parent = addToLoaderTree(root, loaders, loader.getParent());
// 把本人加进去
self = new ClassLoaderWraper(loader);
loaders.put(loader, self);
parent.children.add(self);
self.level = parent.level + 1;
return self;
}
/**
* 以 yml 格局记录树信息
*
* @param buf
* @param wraper
*/
private void traversalLoaderTree(StringBuilder buf, ClassLoaderWraper wraper) {
// 每一层缩进两个空格
char[] prefix = new char[wraper.level * 2];
for (int i = 0; i < prefix.length; i++) {prefix[i] = ' ';
}
buf.append(prefix).append(wraper.name).append(":\n");
buf.append(prefix).append("COUNT:").append(wraper.count).append("\n");
if (wraper.loader instanceof URLClassLoader) {URLClassLoader urlLoader = (URLClassLoader) wraper.loader;
buf.append(prefix).append("URLS:\n");
for (URL url : urlLoader.getURLs()) {buf.append(prefix).append("-").append(url).append("\n");
}
}
if (!wraper.children.isEmpty()) {Collections.sort(wraper.children);
for (ClassLoaderWraper child : wraper.children) {traversalLoaderTree(buf, child);
}
}
}
private static class ClassLoaderWraper implements Comparable<ClassLoaderWraper> {
private ClassLoader loader;
private transient String name;
// 该 loader 间接加载的 class 数量
private int count;
// 不用每次计算 level
private transient int level;
private List<ClassLoaderWraper> children = new ArrayList<ClassLoaderWraper>();
public ClassLoaderWraper(ClassLoader loader) {
this.loader = loader;
this.name = loader == null ? "ROOT" : loader.toString();
this.count++;
}
@Override
public int compareTo(ClassLoaderWraper an) {
int diff = level - an.level;
if (diff != 0) {return level;}
return name.compareTo(an.name);
}
}
}agent installer 将 agent attach 到第 3 方 jvm 中
public class AgentInstaller {
/**
* 将指定 agentJar 附加到 第三方 jvm 过程上，日志输入到 logdir
*
* @param jvmpid
* 第三方 jvm 过程 id
* @param agentJar
* @param logdir
* 输入日志目录
*/
public void install(String jvmpid, File agentJar, File logdir) {StringBuilder buf = new StringBuilder();
logArgsInfo(buf, jvmpid, agentJar, logdir);
VirtualMachine vm = null;
boolean attachSuccess = false;
try {vm = VirtualMachine.attach(jvmpid);
buf.append(vm.getSystemProperties()).append("\n");
attachSuccess = true;
// 将输入目录作为参数传递给 agent
String agentO = URLEncoder.encode(logdir.getAbsolutePath(), "UTF-8");
vm.loadAgent(agentJar.getAbsolutePath(), "o=" + agentO);
}
catch (Throwable e) {if (!attachSuccess) {logError(buf, "获取 JVM 过程失败！请查看 过程 id 指向的过程是否存在、或者是否 JVM 过程！pid=" + jvmpid + "\n", e);
}
else {logError(buf, "将 agent 附加到 JVM 失败！请查看指定 pid 的 JVM 版本，目前只反对 JRE 1.8.0_181！pid=" + jvmpid + "\n", e);
}
}
finally {if (vm != null) {
try {vm.detach();
}
catch (Throwable e) {logError(buf, "vm.detach()执行失败！\n", e);
}
}
writeLogFile(buf, logdir);
System.out.println("执行实现。请查看日志：" + logdir);
}
}
}
public class AgentInstallerFromCmd {public static void main(String[] args) {
try {if (args == null || args.length < 2) {args = loadArgsFromCmd();
}
String pid = args[0];
String logdir = args[1];
URL agentPath = AgentInstaller.class.getProtectionDomain().getCodeSource().getLocation();
new AgentInstaller().install(pid, new File(agentPath.toURI()), new File(logdir));
}
catch (Throwable e) {System.out.println("agent 装置失败！");
e.printStackTrace();}
}
/**
* 从命令行录入 命令参数
*
* @return String[]同命令行参数内容, [0]=jvmpid, [1]=logdir
*/
private static String[] loadArgsFromCmd() {List<VirtualMachineDescriptor> vms = VirtualMachine.list();
for (VirtualMachineDescriptor vmd : vms) {System.out.println("jvm：" + vmd.id() + "\t" + vmd.displayName());
}
String[] args = new String[2];
Scanner scanner = new Scanner(System.in);
try {System.out.println("请输出 指标 Jvm 过程 id：");
args[0] = scanner.nextLine();
System.out.println("请输出 日志输入目录：");
args[1] = scanner.nextLine();
return args;
}
finally {scanner.close();
}
}
}manifest.mf 文件内容
Manifest-Version: 1.0
Bnd-LastModified: 1668494909100
Bundle-ManifestVersion: 2
Bundle-Name: toone-agent
Bundle-SymbolicName: toone-agent
Bundle-Version: 1.0
DynamicImport-Package: *
Agent-Class: toone.agent.loader.AgentMain
Main-Class: toone.AgentInstallerFromCmd
正文完
