前言
学生时代应抱着问题去学习一门语言，例如：在学习java语言的过程中，我遇到过java主方法main里面参数到底是存的什么？还有java语言的Object是如何成为所有类的父类的？java虚拟机到底如何解析字节码文件的？java是纯面向对象编程语言那么它的类是怎样的加载过程？今天我就带着大家一探究竟。
步入正题
首先我们都熟悉DOS界面去执行我们编写的源码，第一步使用javac xxx.java命令,这个命令其实就是调用java的编译器，每一门语言都有自己的编译器，有的源文件是可以直接编译为可执行文件。但是java的编译器只能将源码编译成二进制的字节码文件xxx.class，最后在由java命令调用JVM（java虚拟机）。当我们要使用某个类时，虚拟机就会去加载它的字节码文件，并创建对应的对象，把class文件加载到JVM内存的过程就是类的加载过程。
类的加载过程：
1.加载
加载是类加载过程中的一个阶段，这个阶段会在内存中生成一个代表这个类的 java.lang.Class 对象，作为方法区这个类的各种数据的入口。注意这里不一定非得要从一个 Class 文件获取，这里既可以从 ZIP 包中读取（比如从 jar 包和 war 包中读取），也可以在运行时计算生成（动态代理），也可以由其它文件生成（比如将 JSP 文件转换成对应的 Class 类）。
2.验证
这一阶段的主要目的是为了确保 Class 文件的字节流中包含的信息是否符合当前虚拟机的要求，并且不会危害虚拟机自身的安全。
3.准备
准备阶段是正式为类变量分配内存并设置类变量的初始值阶段，即在方法区中分配这些变量所使用的内存空间。注意这里所说的初始值概念，比如一个类变量定义为：
public static int v = 8080;
实际上变量 v 在准备阶段过后的初始值为 0 而不是 8080，将 v 赋值为 8080 的 put static 指令是程序被编译后，存放于类构造器方法之中。
但是注意如果声明为：
public static final int v = 8080;
在编译阶段会为 v 生成 ConstantValue 属性，在准备阶段虚拟机会根据 ConstantValue 属性将 v赋值为 8080。
4.解析
解析阶段是指虚拟机将常量池中的符号引用替换为直接引用的过程。符号引用就是 class 文件中的：CONSTANT_Class_info、CONSTANT_Field_info、CONSTANT_Method_info等类型的常量。
符号引用： 与虚拟机实现的布局无关，引用的目标并不一定要已经加载到内存中。各种虚拟机实现的内存布局可以各不相同，但是它们能接受的符号引用必须是一致的，因为符号引用的字面量形式明确定义在 Java 虚拟机规范的 Class 文件格式中。
直接引用： 可以是指向目标的指针，相对偏移量或是一个能间接定位到目标的句柄。如果有了直接引用，那引用的目标必定已经在内存中存在。
5.初始化
初始化阶段是类加载最后一个阶段，前面的类加载阶段之后，除了在加载阶段可以自定义类加载器以外，其它操作都由 JVM 主导。到了初始阶段，才开始真正执行类中定义的 Java 程序代码。
初始化阶段是执行类构造器
<client>方法的过程。<client>方法是由编译器自动收集类中的类变
量的赋值操作和静态语句块中的语句合并而成的。虚拟机会保证子<client>方法执行之前，父类
的<client>方法已经执行完毕，如果一个类中没有对静态变量赋值也没有静态语句块，那么编译
器可以不为这个类生成<client>()方法。
以下6种情况不会执行类的初始化
- 通过子类引用父类的静态字段，只会触发父类的初始化，而不会触发子类的初始化。
- 定义对象数组，不会触发该类的初始化。
- 常量在编译期间会存入调用类的常量池中，本质上并没有直接引用定义常量的类，不会触发定义常量所在的类。
- 通过类名获取 Class 对象，不会触发类的初始化。
- 通过 Class.forName 加载指定类时，如果指定参数 initialize 为 false 时，也不会触发类初始化，其实这个参数是告诉虚拟机，是否要对类进行初始化。
- 通过 ClassLoader 默认的 loadClass 方法，也不会触发初始化动作。
类加载器
以上类的加载过程肯定离不开java的类加载器，接下来我们谈谈类加载器，虚拟机设计团队把加载动作放到 JVM 外部实现，以便让应用程序决定如何获取所需的类，JVM 提供了 3 种类加载器：
1.启动类加载器Bootstrap ClassLoader
负责加载 JAVA_HOME\lib 目录中的，或通过-Xbootclasspath 参数指定路径中的，且被虚拟机认可（按文件名识别，如 rt.jar）的类。
2.扩展类加载器(Extension ClassLoader)
负责加载 JAVA_HOME\lib\ext 目录中的，或通过 java.ext.dirs 系统变量指定路径中的类库。
3.应用程序类加载器(Application ClassLoader)
负责加载用户路径（classpath）上的类库。JVM 通过双亲委派模型进行类的加载，当然我们也可以通过继承 java.lang.ClassLoader实现自定义的类加载器。
双亲委派： 在应用程序类加载器中采用了双亲委派机制，如果一个程序需要加载一个类时，此类并不会立马就去执行加载器，而是先去执行父类的类加载器，如果父类还有父类的话，会层层往上执行，如果发现父类的类加载器并不能运行加载的话，在由本类去加载。
当一个类收到了类加载请求，他首先不会尝试自己去加载这个类，而是把这个请求委派给父类去完成，每一个层次类加载器都是如此，因此所有的加载请求都应该传送到启动类加载其中，只有当父类加载器反馈自己无法完成这个请求的时候（在它的加载路径下没有找到所需加载的Class），子类加载器才会尝试自己去加载。
采用双亲委派的一个好处是比如加载位于 rt.jar 包中的类 java.lang.Object，不管是哪个加载器加载这个类，最终都是委托给顶层的启动类加载器进行加载，这样就保证了使用不同的类加载器最终得到的都是同样一个 Object 对象。
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
// 如果 Java 虚拟机已将此加载器记录为具有给定二进制名称的某个类的启动加载器，
//则返回该二进制名称的类。
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
c = parent.loadClass(name, false);//继续查找父类的类加载
} else {
c = findBootstrapClassOrNull(name);//返回启动类加载器加载的类
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
注意点：
1.loadClass（）方法是ClassLoader类实现的，方法中的逻辑就是双亲委派机制
2.运行时加载自己指定的类，那么我们可以直接使用this.getClass().getClassLoder.loadClass(“className”)，这样就可以直接调用ClassLoader的loadClass方法获取到class对象。
3.findClass(String name)
ClassLoader类中并没有实现findClass()方法的具体代码逻辑，取而代之的是抛出ClassNotFoundException异常
4.defineClass(byte[] b, int off, int len)
使用可选的 ProtectionDomain 将 ByteBuffer 转换为 Class 类的实例。如果该域为 null，则将默认域分配给 defineClass(String, byte[], int, int) 的文档中指定的类。这个类必须分析后才能使用。
有关包中定义的第一个类（它确定了包的证书集合）的规则，以及对类名称的限制，都与 defineClass(String, byte[], int, int, ProtectionDomain) 的文档中指定的相同。
调用形式为 cl.defineClass(name, bBuffer, pd) 的此方法所产生的结果与以下语句产生的结果相同 ，注意的是，如果直接调用defineClass()方法生成类的Class对象，这个类的Class对象并没有解析(也可以理解为链接阶段，毕竟解析是链接的最后一步)，其解析操作需要等待初始化阶段进行。
protected Class<?> findClass(String name) throws ClassNotFoundException {
// 获取类的字节数组
byte[] classData = getClassData(name);
if (classData == null) {
throw new ClassNotFoundException();
} else {
//使用defineClass生成class对象
return defineClass(name, classData, 0, classData.length);
}
}
5.URLClassLoader
在创建URLClassPath对象时，会根据传递过来的URL数组中的路径判断是文件还是jar包，即拓展类加载器ExtClassLoader和系统类加载器AppClassLoader，这两个类都继承自URLClassLoader，是sun.misc.Launcher的静态内部类。sun.misc.Launcher主要被系统用于启动主应用程序，ExtClassLoader和AppClassLoader都是由sun.misc.Launcher创建的。
6.类加载器的关系
7.注意class文件的显示加载和隐式加载
JVM加载class文件到内存的方式，显示加载就是直接调用类加载器加载class文件，例如：Class.forName()或者是this.getClass().getClassLoader().loadClass()加载class对象。隐式不会直接在代码里调用类加载器，而是通过虚拟机自动加载到内存中。
源码
package java.lang;
import java.io.InputStream;
import java.io.IOException;
import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.AccessController;
import java.security.AccessControlContext;
import java.security.CodeSource;
import java.security.Policy;
import java.security.PrivilegedAction;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.security.ProtectionDomain;
import java.security.cert.Certificate;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import java.util.Stack;
import java.util.Map;
import java.util.Vector;
import java.util.Hashtable;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;
import sun.misc.CompoundEnumeration;
import sun.misc.Resource;
import sun.misc.URLClassPath;
import sun.misc.VM;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;
import sun.security.util.SecurityConstants;
public abstract class ClassLoader {
private static native void registerNatives();
static {
registerNatives();
}
// The parent class loader for delegation
// Note: VM hardcoded the offset of this field, thus all new fields
// must be added *after* it.
private final ClassLoader parent;
private static class ParallelLoaders {
private ParallelLoaders() {}
// the set of parallel capable loader types
private static final Set<Class<? extends ClassLoader>> loaderTypes =
Collections.newSetFromMap(
new WeakHashMap<Class<? extends ClassLoader>, Boolean>());
static {
synchronized (loaderTypes) { loaderTypes.add(ClassLoader.class); }
}
static boolean register(Class<? extends ClassLoader> c) {
synchronized (loaderTypes) {
if (loaderTypes.contains(c.getSuperclass())) {
// register the class loader as parallel capable
// if and only if all of its super classes are.
// Note: given current classloading sequence, if
// the immediate super class is parallel capable,
// all the super classes higher up must be too.
loaderTypes.add(c);
return true;
} else {
return false;
}
}
}
static boolean isRegistered(Class<? extends ClassLoader> c) {
synchronized (loaderTypes) {
return loaderTypes.contains(c);
}
}
}
// Maps class name to the corresponding lock object when the current
// class loader is parallel capable.
// Note: VM also uses this field to decide if the current class loader
// is parallel capable and the appropriate lock object for class loading.
private final ConcurrentHashMap<String, Object> parallelLockMap;
// Hashtable that maps packages to certs
private final Map <String, Certificate[]> package2certs;
// Shared among all packages with unsigned classes
private static final Certificate[] nocerts = new Certificate[0];
// The classes loaded by this class loader. The only purpose of this table
// is to keep the classes from being GC'ed until the loader is GC'ed.
private final Vector<Class<?>> classes = new Vector<>();
// The "default" domain. Set as the default ProtectionDomain on newly
// created classes.
private final ProtectionDomain defaultDomain =
new ProtectionDomain(new CodeSource(null, (Certificate[]) null),
null, this, null);
// The initiating protection domains for all classes loaded by this loader
private final Set<ProtectionDomain> domains;
// Invoked by the VM to record every loaded class with this loader.
void addClass(Class<?> c) {
classes.addElement(c);
}
// The packages defined in this class loader. Each package name is mapped
// to its corresponding Package object.
// @GuardedBy("itself")
private final HashMap<String, Package> packages = new HashMap<>();
private static Void checkCreateClassLoader() {
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
return null;
}
private ClassLoader(Void unused, ClassLoader parent) {
this.parent = parent;
if (ParallelLoaders.isRegistered(this.getClass())) {
parallelLockMap = new ConcurrentHashMap<>();
package2certs = new ConcurrentHashMap<>();
domains =
Collections.synchronizedSet(new HashSet<ProtectionDomain>());
assertionLock = new Object();
} else {
// no finer-grained lock; lock on the classloader instance
parallelLockMap = null;
package2certs = new Hashtable<>();
domains = new HashSet<>();
assertionLock = this;
}
}
protected ClassLoader(ClassLoader parent) {
this(checkCreateClassLoader(), parent);
}
protected ClassLoader() {
this(checkCreateClassLoader(), getSystemClassLoader());
}
// -- Class --
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
Class<?> c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClassOrNull(name);
}
} catch (ClassNotFoundException e) {
// ClassNotFoundException thrown if class not found
// from the non-null parent class loader
}
if (c == null) {
// If still not found, then invoke findClass in order
// to find the class.
long t1 = System.nanoTime();
c = findClass(name);
// this is the defining class loader; record the stats
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
}
protected Object getClassLoadingLock(String className) {
Object lock = this;
if (parallelLockMap != null) {
Object newLock = new Object();
lock = parallelLockMap.putIfAbsent(className, newLock);
if (lock == null) {
lock = newLock;
}
}
return lock;
}
// This method is invoked by the virtual machine to load a class.
private Class<?> loadClassInternal(String name)
throws ClassNotFoundException
{
// For backward compatibility, explicitly lock on 'this' when
// the current class loader is not parallel capable.
if (parallelLockMap == null) {
synchronized (this) {
return loadClass(name);
}
} else {
return loadClass(name);
}
}
// Invoked by the VM after loading class with this loader.
private void checkPackageAccess(Class<?> cls, ProtectionDomain pd) {
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
if (ReflectUtil.isNonPublicProxyClass(cls)) {
for (Class<?> intf: cls.getInterfaces()) {
checkPackageAccess(intf, pd);
}
return;
}
final String name = cls.getName();
final int i = name.lastIndexOf('.');
if (i != -1) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
sm.checkPackageAccess(name.substring(0, i));
return null;
}
}, new AccessControlContext(new ProtectionDomain[] {pd}));
}
}
domains.add(pd);
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
@Deprecated
protected final Class<?> defineClass(byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(null, b, off, len, null);
}
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(name, b, off, len, null);
}
private ProtectionDomain preDefineClass(String name,
ProtectionDomain pd)
{
if (!checkName(name))
throw new NoClassDefFoundError("IllegalName: " + name);
// Note: Checking logic in java.lang.invoke.MemberName.checkForTypeAlias
// relies on the fact that spoofing is impossible if a class has a name
// of the form "java.*"
if ((name != null) && name.startsWith("java.")) {
throw new SecurityException
("Prohibited package name: " +
name.substring(0, name.lastIndexOf('.')));
}
if (pd == null) {
pd = defaultDomain;
}
if (name != null) checkCerts(name, pd.getCodeSource());
return pd;
}
private String defineClassSourceLocation(ProtectionDomain pd)
{
CodeSource cs = pd.getCodeSource();
String source = null;
if (cs != null && cs.getLocation() != null) {
source = cs.getLocation().toString();
}
return source;
}
private void postDefineClass(Class<?> c, ProtectionDomain pd)
{
if (pd.getCodeSource() != null) {
Certificate certs[] = pd.getCodeSource().getCertificates();
if (certs != null)
setSigners(c, certs);
}
}
protected final Class<?> defineClass(String name, byte[] b, int off, int len,
ProtectionDomain protectionDomain)
throws ClassFormatError
{
protectionDomain = preDefineClass(name, protectionDomain);
String source = defineClassSourceLocation(protectionDomain);
Class<?> c = defineClass1(name, b, off, len, protectionDomain, source);
postDefineClass(c, protectionDomain);
return c;
}
protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,
ProtectionDomain protectionDomain)
throws ClassFormatError
{
int len = b.remaining();
// Use byte[] if not a direct ByteBufer:
if (!b.isDirect()) {
if (b.hasArray()) {
return defineClass(name, b.array(),
b.position() + b.arrayOffset(), len,
protectionDomain);
} else {
// no array, or read-only array
byte[] tb = new byte[len];
b.get(tb); // get bytes out of byte buffer.
return defineClass(name, tb, 0, len, protectionDomain);
}
}
protectionDomain = preDefineClass(name, protectionDomain);
String source = defineClassSourceLocation(protectionDomain);
Class<?> c = defineClass2(name, b, b.position(), len, protectionDomain, source);
postDefineClass(c, protectionDomain);
return c;
}
private native Class<?> defineClass0(String name, byte[] b, int off, int len,
ProtectionDomain pd);
private native Class<?> defineClass1(String name, byte[] b, int off, int len,
ProtectionDomain pd, String source);
private native Class<?> defineClass2(String name, java.nio.ByteBuffer b,
int off, int len, ProtectionDomain pd,
String source);
// true if the name is null or has the potential to be a valid binary name
private boolean checkName(String name) {
if ((name == null) || (name.length() == 0))
return true;
if ((name.indexOf('/') != -1)
|| (!VM.allowArraySyntax() && (name.charAt(0) == '[')))
return false;
return true;
}
private void checkCerts(String name, CodeSource cs) {
int i = name.lastIndexOf('.');
String pname = (i == -1) ? "" : name.substring(0, i);
Certificate[] certs = null;
if (cs != null) {
certs = cs.getCertificates();
}
Certificate[] pcerts = null;
if (parallelLockMap == null) {
synchronized (this) {
pcerts = package2certs.get(pname);
if (pcerts == null) {
package2certs.put(pname, (certs == null? nocerts:certs));
}
}
} else {
pcerts = ((ConcurrentHashMap<String, Certificate[]>)package2certs).
putIfAbsent(pname, (certs == null? nocerts:certs));
}
if (pcerts != null && !compareCerts(pcerts, certs)) {
throw new SecurityException("class \""+ name +
"\"'s signer information does not match signer information of other classes in the same package");
}
}
private boolean compareCerts(Certificate[] pcerts,
Certificate[] certs)
{
// certs can be null, indicating no certs.
if ((certs == null) || (certs.length == 0)) {
return pcerts.length == 0;
}
// the length must be the same at this point
if (certs.length != pcerts.length)
return false;
// go through and make sure all the certs in one array
// are in the other and vice-versa.
boolean match;
for (int i = 0; i < certs.length; i++) {
match = false;
for (int j = 0; j < pcerts.length; j++) {
if (certs[i].equals(pcerts[j])) {
match = true;
break;
}
}
if (!match) return false;
}
// now do the same for pcerts
for (int i = 0; i < pcerts.length; i++) {
match = false;
for (int j = 0; j < certs.length; j++) {
if (pcerts[i].equals(certs[j])) {
match = true;
break;
}
}
if (!match) return false;
}
return true;
}
protected final void resolveClass(Class<?> c) {
resolveClass0(c);
}
private native void resolveClass0(Class<?> c);
protected final Class<?> findSystemClass(String name)
throws ClassNotFoundException
{
ClassLoader system = getSystemClassLoader();
if (system == null) {
if (!checkName(name))
throw new ClassNotFoundException(name);
Class<?> cls = findBootstrapClass(name);
if (cls == null) {
throw new ClassNotFoundException(name);
}
return cls;
}
return system.loadClass(name);
}
private Class<?> findBootstrapClassOrNull(String name)
{
if (!checkName(name)) return null;
return findBootstrapClass(name);
}
// return null if not found
private native Class<?> findBootstrapClass(String name);
protected final Class<?> findLoadedClass(String name) {
if (!checkName(name))
return null;
return findLoadedClass0(name);
}
private native final Class<?> findLoadedClass0(String name);
protected final void setSigners(Class<?> c, Object[] signers) {
c.setSigners(signers);
}
// -- Resource --
public URL getResource(String name) {
URL url;
if (parent != null) {
url = parent.getResource(name);
} else {
url = getBootstrapResource(name);
}
if (url == null) {
url = findResource(name);
}
return url;
}
public Enumeration<URL> getResources(String name) throws IOException {
@SuppressWarnings("unchecked")
Enumeration<URL>[] tmp = (Enumeration<URL>[]) new Enumeration<?>[2];
if (parent != null) {
tmp[0] = parent.getResources(name);
} else {
tmp[0] = getBootstrapResources(name);
}
tmp[1] = findResources(name);
return new CompoundEnumeration<>(tmp);
}
protected URL findResource(String name) {
return null;
}
protected Enumeration<URL> findResources(String name) throws IOException {
return java.util.Collections.emptyEnumeration();
}
@CallerSensitive
protected static boolean registerAsParallelCapable() {
Class<? extends ClassLoader> callerClass =
Reflection.getCallerClass().asSubclass(ClassLoader.class);
return ParallelLoaders.register(callerClass);
}
public static URL getSystemResource(String name) {
ClassLoader system = getSystemClassLoader();
if (system == null) {
return getBootstrapResource(name);
}
return system.getResource(name);
}
public static Enumeration<URL> getSystemResources(String name)
throws IOException
{
ClassLoader system = getSystemClassLoader();
if (system == null) {
return getBootstrapResources(name);
}
return system.getResources(name);
}
private static URL getBootstrapResource(String name) {
URLClassPath ucp = getBootstrapClassPath();
Resource res = ucp.getResource(name);
return res != null ? res.getURL() : null;
}
private static Enumeration<URL> getBootstrapResources(String name)
throws IOException
{
final Enumeration<Resource> e =
getBootstrapClassPath().getResources(name);
return new Enumeration<URL> () {
public URL nextElement() {
return e.nextElement().getURL();
}
public boolean hasMoreElements() {
return e.hasMoreElements();
}
};
}
// Returns the URLClassPath that is used for finding system resources.
static URLClassPath getBootstrapClassPath() {
return sun.misc.Launcher.getBootstrapClassPath();
}
public InputStream getResourceAsStream(String name) {
URL url = getResource(name);
try {
return url != null ? url.openStream() : null;
} catch (IOException e) {
return null;
}
}
public static InputStream getSystemResourceAsStream(String name) {
URL url = getSystemResource(name);
try {
return url != null ? url.openStream() : null;
} catch (IOException e) {
return null;
}
}
// -- Hierarchy --
@CallerSensitive
public final ClassLoader getParent() {
if (parent == null)
return null;
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// Check access to the parent class loader
// If the caller's class loader is same as this class loader,
// permission check is performed.
checkClassLoaderPermission(parent, Reflection.getCallerClass());
}
return parent;
}
@CallerSensitive
public static ClassLoader getSystemClassLoader() {
initSystemClassLoader();
if (scl == null) {
return null;
}
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkClassLoaderPermission(scl, Reflection.getCallerClass());
}
return scl;
}
private static synchronized void initSystemClassLoader() {
if (!sclSet) {
if (scl != null)
throw new IllegalStateException("recursive invocation");
sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
if (l != null) {
Throwable oops = null;
scl = l.getClassLoader();
try {
scl = AccessController.doPrivileged(
new SystemClassLoaderAction(scl));
} catch (PrivilegedActionException pae) {
oops = pae.getCause();
if (oops instanceof InvocationTargetException) {
oops = oops.getCause();
}
}
if (oops != null) {
if (oops instanceof Error) {
throw (Error) oops;
} else {
// wrap the exception
throw new Error(oops);
}
}
}
sclSet = true;
}
}
// Returns true if the specified class loader can be found in this class
// loader's delegation chain.
boolean isAncestor(ClassLoader cl) {
ClassLoader acl = this;
do {
acl = acl.parent;
if (cl == acl) {
return true;
}
} while (acl != null);
return false;
}
// Tests if class loader access requires "getClassLoader" permission
// check. A class loader 'from' can access class loader 'to' if
// class loader 'from' is same as class loader 'to' or an ancestor
// of 'to'. The class loader in a system domain can access
// any class loader.
private static boolean needsClassLoaderPermissionCheck(ClassLoader from,
ClassLoader to)
{
if (from == to)
return false;
if (from == null)
return false;
return !to.isAncestor(from);
}
// Returns the class's class loader, or null if none.
static ClassLoader getClassLoader(Class<?> caller) {
// This can be null if the VM is requesting it
if (caller == null) {
return null;
}
// Circumvent security check since this is package-private
return caller.getClassLoader0();
}
static void checkClassLoaderPermission(ClassLoader cl, Class<?> caller) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// caller can be null if the VM is requesting it
ClassLoader ccl = getClassLoader(caller);
if (needsClassLoaderPermissionCheck(ccl, cl)) {
sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
}
}
}
// The class loader for the system
// @GuardedBy("ClassLoader.class")
private static ClassLoader scl;
// Set to true once the system class loader has been set
// @GuardedBy("ClassLoader.class")
private static boolean sclSet;
// -- Package --
protected Package definePackage(String name, String specTitle,
String specVersion, String specVendor,
String implTitle, String implVersion,
String implVendor, URL sealBase)
throws IllegalArgumentException
{
synchronized (packages) {
Package pkg = getPackage(name);
if (pkg != null) {
throw new IllegalArgumentException(name);
}
pkg = new Package(name, specTitle, specVersion, specVendor,
implTitle, implVersion, implVendor,
sealBase, this);
packages.put(name, pkg);
return pkg;
}
}
protected Package getPackage(String name) {
Package pkg;
synchronized (packages) {
pkg = packages.get(name);
}
if (pkg == null) {
if (parent != null) {
pkg = parent.getPackage(name);
} else {
pkg = Package.getSystemPackage(name);
}
if (pkg != null) {
synchronized (packages) {
Package pkg2 = packages.get(name);
if (pkg2 == null) {
packages.put(name, pkg);
} else {
pkg = pkg2;
}
}
}
}
return pkg;
}
protected Package[] getPackages() {
Map<String, Package> map;
synchronized (packages) {
map = new HashMap<>(packages);
}
Package[] pkgs;
if (parent != null) {
pkgs = parent.getPackages();
} else {
pkgs = Package.getSystemPackages();
}
if (pkgs != null) {
for (int i = 0; i < pkgs.length; i++) {
String pkgName = pkgs[i].getName();
if (map.get(pkgName) == null) {
map.put(pkgName, pkgs[i]);
}
}
}
return map.values().toArray(new Package[map.size()]);
}
// -- Native library access --
protected String findLibrary(String libname) {
return null;
}
static class NativeLibrary {
// opaque handle to native library, used in native code.
long handle;
// the version of JNI environment the native library requires.
private int jniVersion;
// the class from which the library is loaded, also indicates
// the loader this native library belongs.
private final Class<?> fromClass;
// the canonicalized name of the native library.
// or static library name
String name;
// Indicates if the native library is linked into the VM
boolean isBuiltin;
// Indicates if the native library is loaded
boolean loaded;
native void load(String name, boolean isBuiltin);
native long find(String name);
native void unload(String name, boolean isBuiltin);
public NativeLibrary(Class<?> fromClass, String name, boolean isBuiltin) {
this.name = name;
this.fromClass = fromClass;
this.isBuiltin = isBuiltin;
}
protected void finalize() {
synchronized (loadedLibraryNames) {
if (fromClass.getClassLoader() != null && loaded) {
int size = loadedLibraryNames.size();
for (int i = 0; i < size; i++) {
if (name.equals(loadedLibraryNames.elementAt(i))) {
loadedLibraryNames.removeElementAt(i);
break;
}
}
ClassLoader.nativeLibraryContext.push(this);
try {
unload(name, isBuiltin);
} finally {
ClassLoader.nativeLibraryContext.pop();
}
}
}
}
// Invoked in the VM to determine the context class in
// JNI_Load/JNI_Unload
static Class<?> getFromClass() {
return ClassLoader.nativeLibraryContext.peek().fromClass;
}
}
// All native library names we've loaded.
private static Vector<String> loadedLibraryNames = new Vector<>();
// Native libraries belonging to system classes.
private static Vector<NativeLibrary> systemNativeLibraries
= new Vector<>();
// Native libraries associated with the class loader.
private Vector<NativeLibrary> nativeLibraries = new Vector<>();
// native libraries being loaded/unloaded.
private static Stack<NativeLibrary> nativeLibraryContext = new Stack<>();
// The paths searched for libraries
private static String usr_paths[];
private static String sys_paths[];
private static String[] initializePath(String propname) {
String ldpath = System.getProperty(propname, "");
String ps = File.pathSeparator;
int ldlen = ldpath.length();
int i, j, n;
// Count the separators in the path
i = ldpath.indexOf(ps);
n = 0;
while (i >= 0) {
n++;
i = ldpath.indexOf(ps, i + 1);
}
// allocate the array of paths - n :'s = n + 1 path elements
String[] paths = new String[n + 1];
// Fill the array with paths from the ldpath
n = i = 0;
j = ldpath.indexOf(ps);
while (j >= 0) {
if (j - i > 0) {
paths[n++] = ldpath.substring(i, j);
} else if (j - i == 0) {
paths[n++] = ".";
}
i = j + 1;
j = ldpath.indexOf(ps, i);
}
paths[n] = ldpath.substring(i, ldlen);
return paths;
}
// Invoked in the java.lang.Runtime class to implement load and loadLibrary.
static void loadLibrary(Class<?> fromClass, String name,
boolean isAbsolute) {
ClassLoader loader =
(fromClass == null) ? null : fromClass.getClassLoader();
if (sys_paths == null) {
usr_paths = initializePath("java.library.path");
sys_paths = initializePath("sun.boot.library.path");
}
if (isAbsolute) {
if (loadLibrary0(fromClass, new File(name))) {
return;
}
throw new UnsatisfiedLinkError("Can't load library: " + name);
}
if (loader != null) {
String libfilename = loader.findLibrary(name);
if (libfilename != null) {
File libfile = new File(libfilename);
if (!libfile.isAbsolute()) {
throw new UnsatisfiedLinkError(
"ClassLoader.findLibrary failed to return an absolute path: " + libfilename);
}
if (loadLibrary0(fromClass, libfile)) {
return;
}
throw new UnsatisfiedLinkError("Can't load " + libfilename);
}
}
for (int i = 0 ; i < sys_paths.length ; i++) {
File libfile = new File(sys_paths[i], System.mapLibraryName(name));
if (loadLibrary0(fromClass, libfile)) {
return;
}
libfile = ClassLoaderHelper.mapAlternativeName(libfile);
if (libfile != null && loadLibrary0(fromClass, libfile)) {
return;
}
}
if (loader != null) {
for (int i = 0 ; i < usr_paths.length ; i++) {
File libfile = new File(usr_paths[i],
System.mapLibraryName(name));
if (loadLibrary0(fromClass, libfile)) {
return;
}
libfile = ClassLoaderHelper.mapAlternativeName(libfile);
if (libfile != null && loadLibrary0(fromClass, libfile)) {
return;
}
}
}
// Oops, it failed
throw new UnsatisfiedLinkError("no " + name + " in java.library.path");
}
private static native String findBuiltinLib(String name);
private static boolean loadLibrary0(Class<?> fromClass, final File file) {
// Check to see if we're attempting to access a static library
String name = findBuiltinLib(file.getName());
boolean isBuiltin = (name != null);
if (!isBuiltin) {
boolean exists = AccessController.doPrivileged(
new PrivilegedAction<Object>() {
public Object run() {
return file.exists() ? Boolean.TRUE : null;
}})
!= null;
if (!exists) {
return false;
}
try {
name = file.getCanonicalPath();
} catch (IOException e) {
return false;
}
}
ClassLoader loader =
(fromClass == null) ? null : fromClass.getClassLoader();
Vector<NativeLibrary> libs =
loader != null ? loader.nativeLibraries : systemNativeLibraries;
synchronized (libs) {
int size = libs.size();
for (int i = 0; i < size; i++) {
NativeLibrary lib = libs.elementAt(i);
if (name.equals(lib.name)) {
return true;
}
}
synchronized (loadedLibraryNames) {
if (loadedLibraryNames.contains(name)) {
throw new UnsatisfiedLinkError
("Native Library " +
name +
" already loaded in another classloader");
}
int n = nativeLibraryContext.size();
for (int i = 0; i < n; i++) {
NativeLibrary lib = nativeLibraryContext.elementAt(i);
if (name.equals(lib.name)) {
if (loader == lib.fromClass.getClassLoader()) {
return true;
} else {
throw new UnsatisfiedLinkError
("Native Library " +
name +
" is being loaded in another classloader");
}
}
}
NativeLibrary lib = new NativeLibrary(fromClass, name, isBuiltin);
nativeLibraryContext.push(lib);
try {
lib.load(name, isBuiltin);
} finally {
nativeLibraryContext.pop();
}
if (lib.loaded) {
loadedLibraryNames.addElement(name);
libs.addElement(lib);
return true;
}
return false;
}
}
}
// Invoked in the VM class linking code.
static long findNative(ClassLoader loader, String name) {
Vector<NativeLibrary> libs =
loader != null ? loader.nativeLibraries : systemNativeLibraries;
synchronized (libs) {
int size = libs.size();
for (int i = 0; i < size; i++) {
NativeLibrary lib = libs.elementAt(i);
long entry = lib.find(name);
if (entry != 0)
return entry;
}
}
return 0;
}
// -- Assertion management --
final Object assertionLock;
// The default toggle for assertion checking.
// @GuardedBy("assertionLock")
private boolean defaultAssertionStatus = false;
// Maps String packageName to Boolean package default assertion status Note
// that the default package is placed under a null map key. If this field
// is null then we are delegating assertion status queries to the VM, i.e.,
// none of this ClassLoader's assertion status modification methods have
// been invoked.
// @GuardedBy("assertionLock")
private Map<String, Boolean> packageAssertionStatus = null;
// Maps String fullyQualifiedClassName to Boolean assertionStatus If this
// field is null then we are delegating assertion status queries to the VM,
// i.e., none of this ClassLoader's assertion status modification methods
// have been invoked.
// @GuardedBy("assertionLock")
Map<String, Boolean> classAssertionStatus = null;
public void setDefaultAssertionStatus(boolean enabled) {
synchronized (assertionLock) {
if (classAssertionStatus == null)
initializeJavaAssertionMaps();
defaultAssertionStatus = enabled;
}
}
public void setPackageAssertionStatus(String packageName,
boolean enabled) {
synchronized (assertionLock) {
if (packageAssertionStatus == null)
initializeJavaAssertionMaps();
packageAssertionStatus.put(packageName, enabled);
}
}
public void setClassAssertionStatus(String className, boolean enabled) {
synchronized (assertionLock) {
if (classAssertionStatus == null)
initializeJavaAssertionMaps();
classAssertionStatus.put(className, enabled);
}
}
public void clearAssertionStatus() {
synchronized (assertionLock) {
classAssertionStatus = new HashMap<>();
packageAssertionStatus = new HashMap<>();
defaultAssertionStatus = false;
}
}
boolean desiredAssertionStatus(String className) {
synchronized (assertionLock) {
// assert classAssertionStatus != null;
// assert packageAssertionStatus != null;
// Check for a class entry
Boolean result = classAssertionStatus.get(className);
if (result != null)
return result.booleanValue();
// Check for most specific package entry
int dotIndex = className.lastIndexOf(".");
if (dotIndex < 0) { // default package
result = packageAssertionStatus.get(null);
if (result != null)
return result.booleanValue();
}
while(dotIndex > 0) {
className = className.substring(0, dotIndex);
result = packageAssertionStatus.get(className);
if (result != null)
return result.booleanValue();
dotIndex = className.lastIndexOf(".", dotIndex-1);
}
// Return the classloader default
return defaultAssertionStatus;
}
}
// Set up the assertions with information provided by the VM.
// Note: Should only be called inside a synchronized block
private void initializeJavaAssertionMaps() {
// assert Thread.holdsLock(assertionLock);
classAssertionStatus = new HashMap<>();
packageAssertionStatus = new HashMap<>();
AssertionStatusDirectives directives = retrieveDirectives();
for(int i = 0; i < directives.classes.length; i++)
classAssertionStatus.put(directives.classes[i],
directives.classEnabled[i]);
for(int i = 0; i < directives.packages.length; i++)
packageAssertionStatus.put(directives.packages[i],
directives.packageEnabled[i]);
defaultAssertionStatus = directives.deflt;
}
// Retrieves the assertion directives from the VM.
private static native AssertionStatusDirectives retrieveDirectives();
}
class SystemClassLoaderAction
implements PrivilegedExceptionAction<ClassLoader> {
private ClassLoader parent;
SystemClassLoaderAction(ClassLoader parent) {
this.parent = parent;
}
public ClassLoader run() throws Exception {
String cls = System.getProperty("java.system.class.loader");
if (cls == null) {
return parent;
}
Constructor<?> ctor = Class.forName(cls, true, parent)
.getDeclaredConstructor(new Class<?>[] { ClassLoader.class });
ClassLoader sys = (ClassLoader) ctor.newInstance(
new Object[] { parent });
Thread.currentThread().setContextClassLoader(sys);
return sys;
}
}
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