はじめに
以下のようなクラスローダの階層を出力するクラスを作成した。
クラスパスも表示しているが、動的に変わったり、クラスローダの実装によっては取得不可能だったりするので、参考程度。
Java9には未対応。
JigsawやBuiltinClassLoaderを理解して作りなおす必要がある。
bootstrap class loader {id=0}
/opt/java/jdk1.8.0_131/jre/lib/endorsed
/opt/java/jdk1.8.0_131/jre/lib/resources.jar
/opt/java/jdk1.8.0_131/jre/lib/rt.jar
/opt/java/jdk1.8.0_131/jre/lib/sunrsasign.jar
/opt/java/jdk1.8.0_131/jre/lib/jsse.jar
/opt/java/jdk1.8.0_131/jre/lib/jce.jar
/opt/java/jdk1.8.0_131/jre/lib/charsets.jar
/opt/java/jdk1.8.0_131/jre/lib/jfr.jar
/opt/java/jdk1.8.0_131/jre/classes
sun.misc.Launcher$ExtClassLoader {id=405662939}
file:/opt/java/jdk1.8.0_131/jre/lib/ext/sunpkcs11.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/nashorn.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/jaccess.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/sunec.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/zipfs.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/dnsns.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/localedata.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/sunjce_provider.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/jfxrt.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/cldrdata.jar
sun.misc.Launcher$AppClassLoader {context-class-loader=true, id=1252169911, system-class-loader=true}
file:/path/to/classes
package redj.classloader;
import java.io.ByteArrayOutputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.Writer;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.stream.Collectors;
public class ClassLoaders {
private static final Handler[] HANDLERS = {
new BootstrapClassLoaderHandler(),
new URLClassLoaderHandler(),
new UnknownHandler()
};
public static String getDump(ClassLoader classLoader) {
ByteArrayOutputStream out = new ByteArrayOutputStream();
dump(classLoader, out);
return out.toString();
}
public static void dump(ClassLoader classLoader) {
dump(classLoader, System.out);
}
public static void dump(ClassLoader classLoader, OutputStream out) {
PrintWriter writer = new PrintWriter(out, true);
try {
dump(classLoader, writer);
} finally {
writer.flush();
}
}
public static void dump(ClassLoader classLoader, PrintWriter writer) {
for (Handler handler : HANDLERS) {
if (handler.accept(classLoader)) {
handler.handle(classLoader, writer);
break;
}
}
}
public interface Handler {
public boolean accept(ClassLoader classLoader);
public default void handle(ClassLoader classLoader, PrintWriter writer) {
delegateToParent(classLoader, writer);
SortedMap attributes = getAttributes(classLoader);
writer.println(getClassLoaderName(classLoader) + (!attributes.isEmpty() ? " " + attributes : ""));
for (String path : getClasspath(classLoader)) {
writer.println(" " + path);
}
}
public default void delegateToParent(ClassLoader classLoader, PrintWriter writer) {
dump(classLoader.getParent(), writer);
}
public default String getClassLoaderName(ClassLoader classLoader) {
return classLoader.getClass().getName();
}
public default SortedMap getAttributes(ClassLoader classLoader) {
SortedMap<String, String> result = new TreeMap<>();
result.put("id", "" + System.identityHashCode(classLoader));
if (classLoader == ClassLoader.getSystemClassLoader()) {
result.put("system-class-loader", "true");
}
if (classLoader == Thread.currentThread().getContextClassLoader()) {
result.put("context-class-loader", "true");
}
return result;
}
public List<String> getClasspath(ClassLoader classLoader);
}
private static class UnknownHandler implements Handler {
@Override
public boolean accept(ClassLoader classLoader) {
return true;
}
@Override
public List<String> getClasspath(ClassLoader classLoader) {
return Collections.EMPTY_LIST;
}
}
private static class BootstrapClassLoaderHandler implements Handler {
@Override
public boolean accept(ClassLoader classLoader) {
return classLoader == null;
}
@Override
public void delegateToParent(ClassLoader classLoader, PrintWriter writer) {
}
@Override
public String getClassLoaderName(ClassLoader classLoader) {
return "bootstrap class loader";
}
@Override
public List<String> getClasspath(ClassLoader classLoader) {
List<String> result = new ArrayList<>();
result.addAll(splitClassPath(System.getProperty("java.endorsed.dirs")));
result.addAll(splitClassPath(System.getProperty("sun.boot.class.path")));
return result;
}
private List<String> splitClassPath(String classpath) {
List<String> result = Collections.EMPTY_LIST;;
if (classpath != null) {
result = Arrays.asList(classpath.split(System.getProperty("path.separator")));
}
return result;
}
}
private static class URLClassLoaderHandler implements Handler {
@Override
public boolean accept(ClassLoader classLoader) {
return classLoader instanceof URLClassLoader;
}
@Override
public List<String> getClasspath(ClassLoader classLoader) {
return Arrays
.stream(((URLClassLoader) classLoader).getURLs())
.map((url) -> url.toExternalForm())
.collect(Collectors.toList());
}
}
}
Apache TomcatにデプロイしたWebアプリのクラスローダの表示例
ここでは、Apache Tomcat 8.5.15に対して2つのWebアプリtest-webapp-1.warとtest-webapp-2.warをデプロイし、それぞれのアプリからのクラスローダのダンプを出力する。
Webアプリにはそれぞれ以下のようなServletを準備しておき、そのServletにアクセスすることでダンプを出力する。
package redj.classloader;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(urlPatterns = "/dump/*")
public class DumpServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/plain");
ClassLoaders.dump(getClass().getClassLoader(), response.getWriter());
}
}
test-webapp-1.warにアクセスした結果、出力されたダンプは以下。
$ curl http://localhost:8080/test-webapp-1/dump
bootstrap class loader {id=0}
/opt/java/jdk1.8.0_131/jre/lib/endorsed
/opt/java/jdk1.8.0_131/jre/lib/resources.jar
...
sun.misc.Launcher$ExtClassLoader {id=159259014}
file:/opt/java/jdk1.8.0_131/jre/lib/ext/sunpkcs11.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/nashorn.jar
...
sun.misc.Launcher$AppClassLoader {id=1442407170, system-class-loader=true}
file:/opt/apache-tomcat-8.5.15/bin/bootstrap.jar
file:/opt/apache-tomcat-8.5.15/bin/tomcat-juli.jar
java.net.URLClassLoader {id=142257191}
file:/opt/apache-tomcat-8.5.15/lib/
file:/opt/apache-tomcat-8.5.15/lib/tomcat-jdbc.jar
...
org.apache.catalina.loader.ParallelWebappClassLoader {context-class-loader=true, id=819380823}
file:/opt/apache-tomcat-8.5.15/webapps/test-webapp-1/WEB-INF/classes/
test-webapp-2.warにアクセスした結果、出力されたダンプは以下。
$ curl http://localhost:8080/test-webapp-2/dump
bootstrap class loader {id=0}
/opt/java/jdk1.8.0_131/jre/lib/endorsed
/opt/java/jdk1.8.0_131/jre/lib/resources.jar
...
sun.misc.Launcher$ExtClassLoader {id=159259014}
file:/opt/java/jdk1.8.0_131/jre/lib/ext/sunpkcs11.jar
file:/opt/java/jdk1.8.0_131/jre/lib/ext/nashorn.jar
...
sun.misc.Launcher$AppClassLoader {id=1442407170, system-class-loader=true}
file:/opt/apache-tomcat-8.5.15/bin/bootstrap.jar
file:/opt/apache-tomcat-8.5.15/bin/tomcat-juli.jar
java.net.URLClassLoader {id=142257191}
file:/opt/apache-tomcat-8.5.15/lib/
file:/opt/apache-tomcat-8.5.15/lib/tomcat-jdbc.jar
...
org.apache.catalina.loader.ParallelWebappClassLoader {context-class-loader=true, id=1256777078}
file:/opt/apache-tomcat-8.5.15/webapps/test-webapp-2/WEB-INF/classes/
クラスローダのidより、以下のようなツリー構造であることが分かる。
bootstrap class loader {id=0}
`-- ExtClassLoader {id=159259014}
`-- AppClassLoader {id=1442407170}
`-- URLClassLoader {id=142257191}
|-- ParallelWebappClassLoader {id=819380823} // test-webapp-1
`-- ParallelWebappClassLoader {id=1256777078} // test-webapp-2
またParallelWebappClassLoaderはそれぞれcontext-class-loader=trueとマークされており、すなわちコンテキストクラスローダは以下のように設定されていることが分かる。
Webアプリ |
コンテキストクラスローダ |
test-webapp-1 |
ParallelWebappClassLoader {id=819380823} |
test-webapp-2 |
ParallelWebappClassLoader {id=1256777078} |