ClassLoader的屬性
說明：ClassLoader的屬性與其內部類是穿插着創建的，這裏寫在一起了
private static native void registerNatives();
//將ClassLoader類中所有native修飾的方法與C 語言描述的方法對應上相當於做了一次映射
//也側面反映了一次編寫可以處處運行的一種道理，同時相當於基於java與其他語言做了一個適配
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.
// 這個屬性被JVM硬編碼到類的第一個位置，標識委託的父類加載器
// 這麼做的原因可能是爲了方便初始化ClassLoader對象的時候先初始化自己，然後自己再初始化自己擁有的屬性，保障加載順序
private final ClassLoader parent;
/**
* Encapsulates the set of parallel capable loader types.
*/
//這個內部類提供了並行加載類的特性
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); }
}
/**
* Registers the given class loader type as parallel capabale.
* Returns {@code true} is successfully registered; {@code false} if
* loader's super class is not registered.
*/
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;
}
}
}
/**
* Returns {@code true} if the given class loader type is
* registered as parallel capable.
*/
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.
//並行加載的時候爲了保障不會重複加載使用下面的屬性標識加載指定類的時候進行
//鎖定
private final ConcurrentHashMap<String, Object> parallelLockMap;
//這個屬性跟TSL有關，也跟JDK版本有關
private final Map <String, Certificate[]> package2certs;
// Shared among all packages with unsigned classes
// 所有的類都有指定的證書
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.
//
//當前類加載加載的類向量集合，如果類加載器被回收的話這裏面的類也會被回收
private final Vector<Class<?>> classes = new Vector<>();
// The "default" domain. Set as the default ProtectionDomain on newly
// created classes.
// 默認的類的訪問級別
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
//訪問級別集合
private final Set<ProtectionDomain> domains;
// Invoked by the VM to record every loaded class with this loader.
//這裏沒有被顯示調用
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")
// 這裏其實對java類中的包結構做了一次抽象，封裝了Package對象，這個對象也是被類加載器所加載的，
private final HashMap<String, Package> packages = new HashMap<>();
// The class loader for the system
// @GuardedBy("ClassLoader.class")
//這裏定義這個屬性作爲系統類加載器
private static ClassLoader scl;
// Set to true once the system class loader has been set
// @GuardedBy("ClassLoader.class")
//當系統類加載器初始化之後設置初始化狀態
private static boolean sclSet;
// All native library names we've loaded.
//已經加載的本地lib列表
private static Vector<String> loadedLibraryNames = new Vector<>();
// Native libraries belonging to system classes.
// 本地加載lib列表 屬於系統類的
private static Vector<NativeLibrary> systemNativeLibraries
= new Vector<>();
// Native libraries associated with the class loader.
//與當前類加載器相關的本地lib
private Vector<NativeLibrary> nativeLibraries = new Vector<>();
// native libraries being loaded/unloaded.
//已經加載或者沒加載的本地lib
private static Stack<NativeLibrary> nativeLibraryContext = new Stack<>();
// The paths searched for libraries
//搜索的lib目錄
private static String usr_paths[];
private static String sys_paths[];
// -- Assertion management --
//定義一個對象作爲斷言鎖，後續會講到
final Object assertionLock;
// The default toggle for assertion checking.
// @GuardedBy("assertionLock")
//默認斷言狀態
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")
//加載包的時候進行斷言加鎖
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")
//加載包的時候進行斷言加鎖，相當於驗證是否加載了包，
Map<String, Boolean> classAssertionStatus = null;
ClassLoader的構造方法
private ClassLoader(Void unused, ClassLoader parent) {
this.parent = parent;
//如果當前類加載器已經註冊到並行類加載器容器中就初始化對應的屬性
//使用併發安全容器類
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
//使用普通容器集合
parallelLockMap = null;
package2certs = new Hashtable<>();
domains = new HashSet<>();
assertionLock = this;
}
//這裏的構造方法主要操作有兩個
//1.設置當前類加載的父類加載器
//2.根據是否註冊了並行類加載容器初始化對應的容器
}
/**
* Creates a new class loader using the specified parent class loader for
* delegation.
*
* <p> If there is a security manager, its {@link
* SecurityManager#checkCreateClassLoader()
* <tt>checkCreateClassLoader</tt>} method is invoked. This may result in
* a security exception. </p>
*
* @param parent
* The parent class loader
*
* @throws SecurityException
* If a security manager exists and its
* <tt>checkCreateClassLoader</tt> method doesn't allow creation
* of a new class loader.
*
* @since 1.2
*/
protected ClassLoader(ClassLoader parent) {
this(checkCreateClassLoader(), parent);
}
/**
* Creates a new class loader using the <tt>ClassLoader</tt> returned by
* the method {@link #getSystemClassLoader()
* <tt>getSystemClassLoader()</tt>} as the parent class loader.
*
* <p> If there is a security manager, its {@link
* SecurityManager#checkCreateClassLoader()
* <tt>checkCreateClassLoader</tt>} method is invoked. This may result in
* a security exception. </p>
*
* @throws SecurityException
* If a security manager exists and its
* <tt>checkCreateClassLoader</tt> method doesn't allow creation
* of a new class loader.
*/
protected ClassLoader() {
this(checkCreateClassLoader(), getSystemClassLoader());
}ClassLoader執行類加載的過程
/**
* Loads the class with the specified <a href="#name">binary name</a>.
* This method searches for classes in the same manner as the {@link
* #loadClass(String, boolean)} method. It is invoked by the Java virtual
* machine to resolve class references. Invoking this method is equivalent
* to invoking {@link #loadClass(String, boolean) <tt>loadClass(name,
* false)</tt>}. </p>
*
* @param name
* The <a href="#name">binary name</a> of the class
*
* @return The resulting <tt>Class</tt> object
*
* @throws ClassNotFoundException
* If the class was not found
*/
//定義一個重載方法，這裏的加載其實也就是創建Class實例的意思
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
/**
* Loads the class with the specified <a href="#name">binary name</a>. The
* default implementation of this method searches for classes in the
* following order:
*
* <p><ol>
*
* <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
* has already been loaded. </p></li>
*
* <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method
* on the parent class loader. If the parent is <tt>null</tt> the class
* loader built-in to the virtual machine is used, instead. </p></li>
*
* <li><p> Invoke the {@link #findClass(String)} method to find the
* class. </p></li>
*
* </ol>
*
* <p> If the class was found using the above steps, and the
* <tt>resolve</tt> flag is true, this method will then invoke the {@link
* #resolveClass(Class)} method on the resulting <tt>Class</tt> object.
*
* <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link
* #findClass(String)}, rather than this method. </p>
*
* <p> Unless overridden, this method synchronizes on the result of
* {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
* during the entire class loading process.
*
* @param name
* The <a href="#name">binary name</a> of the class
*
* @param resolve
* If <tt>true</tt> then resolve the class
*
* @return The resulting <tt>Class</tt> object
*
* @throws ClassNotFoundException
* If the class could not be found
*/
//看方法的修飾符，
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
//先根據方法名判斷是否已經有類加載器在加載這個類了，
//這裏有兩層含義1.synchronized標明這裏的代碼區是有併發安全問題的
//意思是不能有多個線程同時執行類加載，否則會導致出現同一個類被加載多次
//2.第二層含義就是下面的方法，下面的方法是classLoader提供的並行加載機制，並行加載的時候也需要
//保證同一個類不能被加載多次
synchronized (getClassLoadingLock(name)) {
// First, check if the class has already been loaded
//有上面的加鎖機制之後這裏就可以知道這個類是否被加載了，如果沒有就執行臨界區代碼
Class c = findLoadedClass(name);
if (c == null) {
long t0 = System.nanoTime();
try {
if (parent != null) {
//父類加載器不爲空先由父類加載器加載
c = parent.loadClass(name, false);
} else {
//父類加載器爲空使用根類加載器加載，注意，如果根類加載器加載不到就可能返回空
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();
//這裏如果還麼有找到自己才進行類加載，也就是說雙親委派模式下沒有找到類，就自己加載，自己找類
//這個方法在ClassLoader中是沒有實現的，所以如果父類沒有找到子類必須通過這個方法找類
c = findClass(name);
// this is the defining class loader; record the stats
//這裏可能是進行類加載的一些性能統計
sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
sun.misc.PerfCounter.getFindClasses().increment();
}
}
//resolve 這裏如果是true,進行類的鏈接（驗證，準備，解析）
if (resolve) {
resolveClass(c);
}
return c;
}
}
/**
* Returns the lock object for class loading operations.
* For backward compatibility, the default implementation of this method
* behaves as follows. If this ClassLoader object is registered as
* parallel capable, the method returns a dedicated object associated
* with the specified class name. Otherwise, the method returns this
* ClassLoader object. </p>
*
* @param className
* The name of the to-be-loaded class
*
* @return the lock for class loading operations
*
* @throws NullPointerException
* If registered as parallel capable and <tt>className</tt> is null
*
* @see #loadClass(String, boolean)
*
* @since 1.7
*/
//這裏使用併發安全容器類實現類加載過程的鎖機制，保證線程安全
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;
}ClassLoader查找類的過程
說明:這裏查找類的過程是由JVM底層實現的，比較能看懂，不做解析
// return null if not found
private native Class findBootstrapClass(String name);
/**
* Returns the class with the given <a href="#name">binary name</a> if this
* loader has been recorded by the Java virtual machine as an initiating
* loader of a class with that <a href="#name">binary name</a>. Otherwise
* <tt>null</tt> is returned. </p>
*
* @param name
* The <a href="#name">binary name</a> of the class
*
* @return The <tt>Class</tt> object, or <tt>null</tt> if the class has
* not been loaded
*
* @since 1.1
*/
protected final Class<?> findLoadedClass(String name) {
if (!checkName(name))
return null;
return findLoadedClass0(name);
}
private native final Class findLoadedClass0(String name);
/**
* Links the specified class. This (misleadingly named) method may be
* used by a class loader to link a class. If the class <tt>c</tt> has
* already been linked, then this method simply returns. Otherwise, the
* class is linked as described in the "Execution" chapter of
* <cite>The Java™ Language Specification</cite>.
* </p>
*
* @param c
* The class to link
*
* @throws NullPointerException
* If <tt>c</tt> is <tt>null</tt>.
*
* @see #defineClass(String, byte[], int, int)
*/
protected final void resolveClass(Class<?> c) {
resolveClass0(c);
}
private native void resolveClass0(Class c);ClassLoader定義包的過程
說明：定義類之前先定義包，這裏的包的意思跟命名空間類似，可以搜一下java中的命名空間
/**
* Defines a package by name in this <tt>ClassLoader</tt>. This allows
* class loaders to define the packages for their classes. Packages must
* be created before the class is defined, and package names must be
* unique within a class loader and cannot be redefined or changed once
* created. </p>
*
* @param name
* The package name
*
* @param specTitle
* The specification title
*
* @param specVersion
* The specification version
*
* @param specVendor
* The specification vendor
*
* @param implTitle
* The implementation title
*
* @param implVersion
* The implementation version
*
* @param implVendor
* The implementation vendor
*
* @param sealBase
* If not <tt>null</tt>, then this package is sealed with
* respect to the given code source {@link java.net.URL
* <tt>URL</tt>} object. Otherwise, the package is not sealed.
*
* @return The newly defined <tt>Package</tt> object
*
* @throws IllegalArgumentException
* If package name duplicates an existing package either in this
* class loader or one of its ancestors
*
* @since 1.2
*/
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;
}
}
/**
* Returns a <tt>Package</tt> that has been defined by this class loader
* or any of its ancestors. </p>
*
* @param name
* The package name
*
* @return The <tt>Package</tt> corresponding to the given name, or
* <tt>null</tt> if not found
*
* @since 1.2
*/
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;
}
/**
* Returns all of the <tt>Packages</tt> defined by this class loader and
* its ancestors. </p>
*
* @return The array of <tt>Package</tt> objects defined by this
* <tt>ClassLoader</tt>
*
* @since 1.2
*/
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()]);
}ClassLoader定義/創建類的過程
說明：這裏的一些方法是類加載的一些核心內容
/**
* Converts an array of bytes into an instance of class <tt>Class</tt>.
* Before the <tt>Class</tt> can be used it must be resolved. This method
* is deprecated in favor of the version that takes a <a
* href="#name">binary name</a> as its first argument, and is more secure.
*
* @param b
* The bytes that make up the class data. The bytes in positions
* <tt>off</tt> through <tt>off len-1</tt> should have the format
* of a valid class file as defined by
* <cite>The Java™ Virtual Machine Specification</cite>.
*
* @param off
* The start offset in <tt>b</tt> of the class data
*
* @param len
* The length of the class data
*
* @return The <tt>Class</tt> object that was created from the specified
* class data
*
* @throws ClassFormatError
* If the data did not contain a valid class
*
* @throws IndexOutOfBoundsException
* If either <tt>off</tt> or <tt>len</tt> is negative, or if
* <tt>off len</tt> is greater than <tt>b.length</tt>.
*
* @throws SecurityException
* If an attempt is made to add this class to a package that
* contains classes that were signed by a different set of
* certificates than this class, or if an attempt is made
* to define a class in a package with a fully-qualified name
* that starts with "{@code java.}".
*
* @see #loadClass(String, boolean)
* @see #resolveClass(Class)
*
* @deprecated Replaced by {@link #defineClass(String, byte[], int, int)
* defineClass(String, byte[], int, int)}
*/
//注意這裏的方法就是類加載過程中鏈接階段要做的事情
@Deprecated
protected final Class<?> defineClass(byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(null, b, off, len, null);
}
/**
* Converts an array of bytes into an instance of class <tt>Class</tt>.
* Before the <tt>Class</tt> can be used it must be resolved.
*
* <p> This method assigns a default {@link java.security.ProtectionDomain
* <tt>ProtectionDomain</tt>} to the newly defined class. The
* <tt>ProtectionDomain</tt> is effectively granted the same set of
* permissions returned when {@link
* java.security.Policy#getPermissions(java.security.CodeSource)
* <tt>Policy.getPolicy().getPermissions(new CodeSource(null, null))</tt>}
* is invoked. The default domain is created on the first invocation of
* {@link #defineClass(String, byte[], int, int) <tt>defineClass</tt>},
* and re-used on subsequent invocations.
*
* <p> To assign a specific <tt>ProtectionDomain</tt> to the class, use
* the {@link #defineClass(String, byte[], int, int,
* java.security.ProtectionDomain) <tt>defineClass</tt>} method that takes a
* <tt>ProtectionDomain</tt> as one of its arguments. </p>
*
* @param name
* The expected <a href="#name">binary name</a> of the class, or
* <tt>null</tt> if not known
*
* @param b
* The bytes that make up the class data. The bytes in positions
* <tt>off</tt> through <tt>off len-1</tt> should have the format
* of a valid class file as defined by
* <cite>The Java™ Virtual Machine Specification</cite>.
*
* @param off
* The start offset in <tt>b</tt> of the class data
*
* @param len
* The length of the class data
*
* @return The <tt>Class</tt> object that was created from the specified
* class data.
*
* @throws ClassFormatError
* If the data did not contain a valid class
*
* @throws IndexOutOfBoundsException
* If either <tt>off</tt> or <tt>len</tt> is negative, or if
* <tt>off len</tt> is greater than <tt>b.length</tt>.
*
* @throws SecurityException
* If an attempt is made to add this class to a package that
* contains classes that were signed by a different set of
* certificates than this class (which is unsigned), or if
* <tt>name</tt> begins with "<tt>java.</tt>".
*
* @see #loadClass(String, boolean)
* @see #resolveClass(Class)
* @see java.security.CodeSource
* @see java.security.SecureClassLoader
*
* @since 1.1
*/
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(name, b, off, len, null);
}
/**
* Converts an array of bytes into an instance of class <tt>Class</tt>,
* with an optional <tt>ProtectionDomain</tt>. If the domain is
* <tt>null</tt>, then a default domain will be assigned to the class as
* specified in the documentation for {@link #defineClass(String, byte[],
* int, int)}. Before the class can be used it must be resolved.
*
* <p> The first class defined in a package determines the exact set of
* certificates that all subsequent classes defined in that package must
* contain. The set of certificates for a class is obtained from the
* {@link java.security.CodeSource <tt>CodeSource</tt>} within the
* <tt>ProtectionDomain</tt> of the class. Any classes added to that
* package must contain the same set of certificates or a
* <tt>SecurityException</tt> will be thrown. Note that if
* <tt>name</tt> is <tt>null</tt>, this check is not performed.
* You should always pass in the <a href="#name">binary name</a> of the
* class you are defining as well as the bytes. This ensures that the
* class you are defining is indeed the class you think it is.
*
* <p> The specified <tt>name</tt> cannot begin with "<tt>java.</tt>", since
* all classes in the "<tt>java.*</tt> packages can only be defined by the
* bootstrap class loader. If <tt>name</tt> is not <tt>null</tt>, it
* must be equal to the <a href="#name">binary name</a> of the class
* specified by the byte array "<tt>b</tt>", otherwise a {@link
* <tt>NoClassDefFoundError</tt>} will be thrown. </p>
*
* @param name
* The expected <a href="#name">binary name</a> of the class, or
* <tt>null</tt> if not known
*
* @param b
* The bytes that make up the class data. The bytes in positions
* <tt>off</tt> through <tt>off len-1</tt> should have the format
* of a valid class file as defined by
* <cite>The Java™ Virtual Machine Specification</cite>.
*
* @param off
* The start offset in <tt>b</tt> of the class data
*
* @param len
* The length of the class data
*
* @param protectionDomain
* The ProtectionDomain of the class
*
* @return The <tt>Class</tt> object created from the data,
* and optional <tt>ProtectionDomain</tt>.
*
* @throws ClassFormatError
* If the data did not contain a valid class
*
* @throws NoClassDefFoundError
* If <tt>name</tt> is not equal to the <a href="#name">binary
* name</a> of the class specified by <tt>b</tt>
*
* @throws IndexOutOfBoundsException
* If either <tt>off</tt> or <tt>len</tt> is negative, or if
* <tt>off len</tt> is greater than <tt>b.length</tt>.
*
* @throws SecurityException
* If an attempt is made to add this class to a package that
* contains classes that were signed by a different set of
* certificates than this class, or if <tt>name</tt> begins with
* "<tt>java.</tt>".
*/
protected final Class<?> defineClass(String name, byte[] b, int off, int len,
ProtectionDomain protectionDomain)
throws ClassFormatError
{
//1.鏈接過程中的第一步1.1驗證
protectionDomain = preDefineClass(name, protectionDomain);
Class c = null;
//1.鏈接過程中的第二步1.2準備，這裏就是加載class字節碼了
String source = defineClassSourceLocation(protectionDomain);
//1.鏈接過程中的第三步1.3解析，這裏就是通過native方法進行具體的解析了
try {
c = defineClass1(name, b, off, len, protectionDomain, source);
} catch (ClassFormatError cfe) {
c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
source);
}
//對類進行簽名
postDefineClass(c, protectionDomain);
return c;
}
/* Determine protection domain, and check that:
- not define java.* class,
- signer of this class matches signers for the rest of the classes in
package.
*/
private ProtectionDomain preDefineClass(String name,
ProtectionDomain pd)
{
if (!checkName(name))
throw new NoClassDefFoundError("IllegalName: " name);
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 Class defineTransformedClass(String name, byte[] b, int off, int len,
ProtectionDomain pd,
ClassFormatError cfe, String source)
throws ClassFormatError
{
// Class format error - try to transform the bytecode and
// define the class again
//
ClassFileTransformer[] transformers =
ClassFileTransformer.getTransformers();
Class c = null;
if (transformers != null) {
for (ClassFileTransformer transformer : transformers) {
try {
// Transform byte code using transformer
byte[] tb = transformer.transform(b, off, len);
c = defineClass1(name, tb, 0, tb.length,
pd, source);
break;
} catch (ClassFormatError cfe2) {
// If ClassFormatError occurs, try next transformer
}
}
}
// Rethrow original ClassFormatError if unable to transform
// bytecode to well-formed
//
if (c == null)
throw cfe;
return c;
}
private void postDefineClass(Class c, ProtectionDomain pd)
{
if (pd.getCodeSource() != null) {
Certificate certs[] = pd.getCodeSource().getCertificates();
if (certs != null)
setSigners(c, certs);
}
}
ClassLoader總結
- ClassLoader展現了雙親委派模式的實現機制
- ClassLoader實現了大概的類加載過程（加載和鏈接階段）
- ClassLoader在加載過程中涉及到了安全管理，類簽名，lib掃描，包定義，類的訪問權限，併發加載的安全性
- ClassLoader源碼總共分爲下面5大部分，由於是偏底層的類，所以代碼會緊湊點，不好閱讀，但是比較穩定
1.Class 的定義和加載
2.Hierarchy 類的層次結構
3.Resource 定義和加載
4.Native library access （lib包訪問）
5.Package 定義
上面只是根據ClassLoader源碼做了一些簡單的解析，更具體的可以看下面的鏈接：https://blog.csdn.net/m0_38075425/article/details/81627349https://www.cnblogs.com/czwbig/p/11127222.html
架構設計@工程設計@服務穩定性之路
