针对 jdk1.8，jdk9 以后就没有该文件了。
http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/raw-file/8477fd88653c/src/share/classes/sun/misc/Launcher.java
public class Launcher {
// 自定义通讯协议设计
private static URLStreamHandlerFactory factory = new Launcher.Factory();
// 不伦不类，说你是单例你却有个公有的构造方法
private static Launcher launcher = new Launcher();
// Bootstrap classloader 加载的路径
private static String bootClassPath = System.getProperty("sun.boot.class.path");
// Application classloader
private ClassLoader loader;
// 没有用到
private static URLStreamHandler fileHandler;
// ...
}
Launcher 构造方法
public Launcher() {
// 创建 ExtClassLoader
ClassLoader extcl;
try {
extcl = ExtClassLoader.getExtClassLoader();
} catch (IOException e) {
throw new InternalError("Could not create extension class loader", e);
}
try {
// 创建 AppClassLoader
// AppClassLoader 的爹设置为 ExtClassLoader
loader = AppClassLoader.getAppClassLoader(extcl);
} catch (IOException e) {
throw new InternalError("Could not create application class loader", e);
}
// 将线程上下文加载器设置为 AppClassLoader
Thread.currentThread().setContextClassLoader(loader);
// Finally, install a security manager if requested
String s = System.getProperty("java.security.manager");
if (s != null) {
SecurityManager sm = null;
if ("".equals(s) || "default".equals(s)) {
sm = new java.lang.SecurityManager();
} else {
try {
sm = (SecurityManager) loader.loadClass(s).newInstance();
} catch (IllegalAccessException | InstantiationException | ClassNotFoundException | ClassCastException e) {
}
}
if (sm != null) {
System.setSecurityManager(sm);
} else {
throw new InternalError("Could not create SecurityManager: " + s);
}
}
}
BootClassPathHolder 私有内部类，也是一种线程安全的单例实现模式。
private static class BootClassPathHolder {
static final URLClassPath bcp;
static {
URL[] urls;
if (bootClassPath != null) {
urls = AccessController.doPrivileged(
new PrivilegedAction<URL[]>() {
@Override
public URL[] run() {
File[] classPath = getClassPath(bootClassPath);
// 加快索引速度，JDK9 以后移除了。
int len = classPath.length;
Set<File> seenDirs = new HashSet<File>();
for (int i = 0; i < len; i++) {
File curEntry = classPath[i];
// Negative test used to properly handle nonexistent jars on boot class path
if (!curEntry.isDirectory()) {
curEntry = curEntry.getParentFile();
}
if (curEntry != null && seenDirs.add(curEntry)) {
MetaIndex.registerDirectory(curEntry);
}
}
return pathToURLs(classPath);
}
}
);
} else {
urls = new URL[0];
}
bcp = new URLClassPath(urls, factory, null);
bcp.initLookupCache(null);
}
}
可以看到 getAppClassLoader 每次都是重新 new 一个 AppClassLoader。
static class AppClassLoader extends URLClassLoader {
// 并行加载
static {
ClassLoader.registerAsParallelCapable();
}
public static ClassLoader getAppClassLoader(final ClassLoader extcl) throws IOException {
final String s = System.getProperty("java.class.path");
final File[] path = (s == null) ? new File[0] : getClassPath(s);
return AccessController.doPrivileged(
new PrivilegedAction<AppClassLoader>() {
@Override
public AppClassLoader run() {
URL[] urls = (s == null) ? new URL[0] : pathToURLs(path);
return new AppClassLoader(urls, extcl);
}
});
}
final URLClassPath ucp;
AppClassLoader(URL[] urls, ClassLoader parent) {
super(urls, parent, factory);
ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
ucp.initLookupCache(this);
}
@Override
public Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
int i = name.lastIndexOf('.');
if (i != -1) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
sm.checkPackageAccess(name.substring(0, i));
}
}
// 在缓存中查找
if (ucp.knownToNotExist(name)) {
Class<?> c = findLoadedClass(name);
if (c != null) {
if (resolve) {
resolveClass(c);
}
return c;
}
throw new ClassNotFoundException(name);
}
return (super.loadClass(name, resolve));
}
// ...
}
整个 JVM 内只有一个 ExtClassLoader
static class ExtClassLoader extends URLClassLoader {
static {
ClassLoader.registerAsParallelCapable();
}
// 单例
private static volatile ExtClassLoader instance = null;
public static ExtClassLoader getExtClassLoader() throws IOException {
if (instance == null) {
synchronized (ExtClassLoader.class) {
if (instance == null) {
instance = createExtClassLoader();
}
}
}
return instance;
}
private static ExtClassLoader createExtClassLoader() throws IOException {
try {
return AccessController.doPrivileged(
new PrivilegedExceptionAction<ExtClassLoader>() {
@Override
public ExtClassLoader run() throws IOException {
final File[] dirs = getExtDirs();
int len = dirs.length;
for (int i = 0; i < len; i++) {
MetaIndex.registerDirectory(dirs[i]);
}
return new ExtClassLoader(dirs);
}
});
} catch (java.security.PrivilegedActionException e) {
throw (IOException) e.getException();
}
}
public ExtClassLoader(File[] dirs) throws IOException {
super(getExtURLs(dirs), null, factory);
SharedSecrets.getJavaNetAccess().
getURLClassPath(this).initLookupCache(this);
}
// ...
}
测试
@Test
public void test3() {
System.out.println(new Launcher());
System.out.println(new Launcher());
System.out.println(new Launcher().getClassLoader());
System.out.println(new Launcher().getClassLoader());
System.out.println(new Launcher().getClassLoader().getParent());
System.out.println(new Launcher().getClassLoader().getParent());
}
/* 输出
sun.misc.Launcher@18ef96
sun.misc.Launcher@6956de9
sun.misc.Launcher$AppClassLoader@769c9116
sun.misc.Launcher$AppClassLoader@6aceb1a5
sun.misc.Launcher$ExtClassLoader@61e4705b
sun.misc.Launcher$ExtClassLoader@61e4705b
*/
System.getProperty("sun.boot.class.path")
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/resources.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/rt.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/sunrsasign.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jsse.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jce.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/charsets.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfr.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/classes
System.getProperty("java.ext.dirs")
/Users/N3verL4nd/Library/Java/Extensions
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext
/Library/Java/Extensions
/Network/Library/Java/Extensions
/System/Library/Java/Extensions
/usr/lib/java
System.getProperty("java.class.path")
idea 相关
/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar
/Applications/IntelliJ IDEA.app/Contents/plugins/junit/lib/junit-rt.jar
/Applications/IntelliJ IDEA.app/Contents/plugins/junit/lib/junit5-rt.jar
/Users/N3verL4nd/Library/Caches/IntelliJIdea2019.2/captureAgent/debugger-agent.jar
jre 相关
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/charsets.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/deploy.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/javaws.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jce.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfr.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jfxswt.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/jsse.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/management-agent.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/plugin.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/resources.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/rt.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/ant-javafx.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/dt.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/javafx-mx.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/jconsole.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/packager.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/sa-jdi.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/lib/tools.jar
jre ext
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/cldrdata.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/dnsns.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jaccess.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/jfxrt.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/localedata.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/nashorn.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunec.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar
/Library/Java/JavaVirtualMachines/jdk1.8.0_211.jdk/Contents/Home/jre/lib/ext/zipfs.jar
项目相关
/Users/N3verL4nd/Desktop/MTThriftStudy/target/test-classes
/Users/N3verL4nd/Desktop/MTThriftStudy/target/classes
pom 配置的包
/Users/N3verL4nd/.m2/repository/org/springframework/spring-webmvc/5.0.8.RELEASE/spring-webmvc-5.0.8.RELEASE.jar
/Users/N3verL4nd/.m2/repository/org/springframework/spring-aop/5.0.8.RELEASE/spring-aop-5.0.8.RELEASE.jar
...
