Poison

Class Reloading

对于同一个类加载器实例,如果我们通过 ClassLoader.loadClassClass.forName 多次去加载同一个全限定类名的类,那么我们得到的是该类对象的同一个实例,我们可以通过源码及单元测试来验证,先看看相关源码:

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/**
* 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>}.
*
* @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
*/
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:
*
* <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
*/
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;
}
}

/**
* 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.
*
* @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);

可以看出 ClassLoader.loadClass 内部调用了 findLoadedClass(name) 进行了类是否已经加载的判断,对于相同 nameClass 实例,只会返回首次加载的 Class 实例。再看看 Class.forName 的源码:

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/**
* Returns the {@code Class} object associated with the class or
* interface with the given string name. Invoking this method is
* equivalent to:
*
* <blockquote>
* {@code Class.forName(className, true, currentLoader)}
* </blockquote>
*
* where {@code currentLoader} denotes the defining class loader of
* the current class.
*
* <p> For example, the following code fragment returns the
* runtime {@code Class} descriptor for the class named
* {@code java.lang.Thread}:
*
* <blockquote>
* {@code Class t = Class.forName("java.lang.Thread")}
* </blockquote>
* <p>
* A call to {@code forName("X")} causes the class named
* {@code X} to be initialized.
*
* @param className the fully qualified name of the desired class.
* @return the {@code Class} object for the class with the
* specified name.
* @exception LinkageError if the linkage fails
* @exception ExceptionInInitializerError if the initialization provoked
* by this method fails
* @exception ClassNotFoundException if the class cannot be located
*/
@CallerSensitive
public static Class<?> forName(String className)
throws ClassNotFoundException {
Class<?> caller = Reflection.getCallerClass();
return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
}

/** Called after security check for system loader access checks have been made. */
private static native Class<?> forName0(String name, boolean initialize,
ClassLoader loader,
Class<?> caller)
throws ClassNotFoundException;

看看 Class.forName0 的原生实现 Class.c:

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JNIEXPORT jclass JNICALL
Java_java_lang_Class_forName0(JNIEnv *env, jclass this, jstring classname,
jboolean initialize, jobject loader)
{
char *clname;
jclass cls = 0;
char buf[128];
jsize len;
jsize unicode_len;

if (classname == NULL) {
JNU_ThrowNullPointerException(env, 0);
return 0;
}

len = (*env)->GetStringUTFLength(env, classname);
unicode_len = (*env)->GetStringLength(env, classname);
if (len >= (jsize)sizeof(buf)) {
clname = malloc(len + 1);
if (clname == NULL) {
JNU_ThrowOutOfMemoryError(env, NULL);
return NULL;
}
} else {
clname = buf;
}
(*env)->GetStringUTFRegion(env, classname, 0, unicode_len, clname);

if (VerifyFixClassname(clname) == JNI_TRUE) {
/* slashes present in clname, use name b4 translation for exception */
(*env)->GetStringUTFRegion(env, classname, 0, unicode_len, clname);
JNU_ThrowClassNotFoundException(env, clname);
goto done;
}

if (!VerifyClassname(clname, JNI_TRUE)) { /* expects slashed name */
JNU_ThrowClassNotFoundException(env, clname);
goto done;
}

cls = JVM_FindClassFromClassLoader(env, clname, initialize,
loader, JNI_FALSE);

done:
if (clname != buf) {
free(clname);
}
return cls;
}

其中 JVM_FindClassFromClassLoader 方法实现位于 jvm.cpp:

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JVM_ENTRY(jclass, JVM_FindClassFromClassLoader(JNIEnv* env, const char* name,
jboolean init, jobject loader,
jboolean throwError))
JVMWrapper3("JVM_FindClassFromClassLoader %s throw %s", name,
throwError ? "error" : "exception");
// Java libraries should ensure that name is never null...
if (name == NULL || (int)strlen(name) > Symbol::max_length()) {
// It's impossible to create this class; the name cannot fit
// into the constant pool.
if (throwError) {
THROW_MSG_0(vmSymbols::java_lang_NoClassDefFoundError(), name);
} else {
THROW_MSG_0(vmSymbols::java_lang_ClassNotFoundException(), name);
}
}
TempNewSymbol h_name = SymbolTable::new_symbol(name, CHECK_NULL);
Handle h_loader(THREAD, JNIHandles::resolve(loader));
jclass result = find_class_from_class_loader(env, h_name, init, h_loader,
Handle(), throwError, THREAD);

if (TraceClassResolution && result != NULL) {
trace_class_resolution(java_lang_Class::as_Klass(JNIHandles::resolve_non_null(result)));
}
return result;
JVM_END

jclass find_class_from_class_loader(JNIEnv* env, Symbol* name, jboolean init, Handle loader, Handle protection_domain, jboolean throwError, TRAPS) {
// Security Note:
// The Java level wrapper will perform the necessary security check allowing
// us to pass the NULL as the initiating class loader.
Klass* klass = SystemDictionary::resolve_or_fail(name, loader, protection_domain, throwError != 0, CHECK_NULL);

KlassHandle klass_handle(THREAD, klass);
// Check if we should initialize the class
if (init && klass_handle->oop_is_instance()) {
klass_handle->initialize(CHECK_NULL);
}
return (jclass) JNIHandles::make_local(env, klass_handle->java_mirror());
}

继续跟踪源码至 systemDictionary.cpp:

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Klass* SystemDictionary::resolve_or_fail(Symbol* class_name, Handle class_loader, Handle protection_domain, bool throw_error, TRAPS) {
Klass* klass = resolve_or_null(class_name, class_loader, protection_domain, THREAD);
if (HAS_PENDING_EXCEPTION || klass == NULL) {
KlassHandle k_h(THREAD, klass);
// can return a null klass
klass = handle_resolution_exception(class_name, class_loader, protection_domain, throw_error, k_h, THREAD);
}
return klass;
}

Klass* SystemDictionary::resolve_or_null(Symbol* class_name, Handle class_loader, Handle protection_domain, TRAPS) {
assert(!THREAD->is_Compiler_thread(),
err_msg("can not load classes with compiler thread: class=%s, classloader=%s",
class_name->as_C_string(),
class_loader.is_null() ? "null" : class_loader->klass()->name()->as_C_string()));
if (FieldType::is_array(class_name)) {
return resolve_array_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL);
} else if (FieldType::is_obj(class_name)) {
ResourceMark rm(THREAD);
// Ignore wrapping L and ;.
TempNewSymbol name = SymbolTable::new_symbol(class_name->as_C_string() + 1,
class_name->utf8_length() - 2, CHECK_NULL);
return resolve_instance_class_or_null(name, class_loader, protection_domain, CHECK_NULL);
} else {
return resolve_instance_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL);
}
}

Klass* SystemDictionary::resolve_instance_class_or_null(Symbol* name,
Handle class_loader,
Handle protection_domain,
TRAPS) {
assert(name != NULL && !FieldType::is_array(name) &&
!FieldType::is_obj(name), "invalid class name");

Ticks class_load_start_time = Ticks::now();

// UseNewReflection
// Fix for 4474172; see evaluation for more details
class_loader = Handle(THREAD, java_lang_ClassLoader::non_reflection_class_loader(class_loader()));
ClassLoaderData *loader_data = register_loader(class_loader, CHECK_NULL);

// Do lookup to see if class already exist and the protection domain
// has the right access
// This call uses find which checks protection domain already matches
// All subsequent calls use find_class, and set has_loaded_class so that
// before we return a result we call out to java to check for valid protection domain
// to allow returning the Klass* and add it to the pd_set if it is valid
unsigned int d_hash = dictionary()->compute_hash(name, loader_data);
int d_index = dictionary()->hash_to_index(d_hash);
Klass* probe = dictionary()->find(d_index, d_hash, name, loader_data,
protection_domain, THREAD);
if (probe != NULL) return probe;


// Non-bootstrap class loaders will call out to class loader and
// define via jvm/jni_DefineClass which will acquire the
// class loader object lock to protect against multiple threads
// defining the class in parallel by accident.
// This lock must be acquired here so the waiter will find
// any successful result in the SystemDictionary and not attempt
// the define
// ParallelCapable Classloaders and the bootstrap classloader,
// or all classloaders with UnsyncloadClass do not acquire lock here
bool DoObjectLock = true;
if (is_parallelCapable(class_loader)) {
DoObjectLock = false;
}

unsigned int p_hash = placeholders()->compute_hash(name, loader_data);
int p_index = placeholders()->hash_to_index(p_hash);

// Class is not in SystemDictionary so we have to do loading.
// Make sure we are synchronized on the class loader before we proceed
Handle lockObject = compute_loader_lock_object(class_loader, THREAD);
check_loader_lock_contention(lockObject, THREAD);
ObjectLocker ol(lockObject, THREAD, DoObjectLock);

// Check again (after locking) if class already exist in SystemDictionary
bool class_has_been_loaded = false;
bool super_load_in_progress = false;
bool havesupername = false;
instanceKlassHandle k;
PlaceholderEntry* placeholder;
Symbol* superclassname = NULL;

{
MutexLocker mu(SystemDictionary_lock, THREAD);
Klass* check = find_class(d_index, d_hash, name, loader_data);
if (check != NULL) {
// Klass is already loaded, so just return it
class_has_been_loaded = true;
k = instanceKlassHandle(THREAD, check);
} else {
placeholder = placeholders()->get_entry(p_index, p_hash, name, loader_data);
if (placeholder && placeholder->super_load_in_progress()) {
super_load_in_progress = true;
if (placeholder->havesupername() == true) {
superclassname = placeholder->supername();
havesupername = true;
}
}
}
}

// If the class is in the placeholder table, class loading is in progress
if (super_load_in_progress && havesupername==true) {
k = SystemDictionary::handle_parallel_super_load(name, superclassname,
class_loader, protection_domain, lockObject, THREAD);
if (HAS_PENDING_EXCEPTION) {
return NULL;
}
if (!k.is_null()) {
class_has_been_loaded = true;
}
}

bool throw_circularity_error = false;
if (!class_has_been_loaded) {
bool load_instance_added = false;

// add placeholder entry to record loading instance class
// Five cases:
// All cases need to prevent modifying bootclasssearchpath
// in parallel with a classload of same classname
// Redefineclasses uses existence of the placeholder for the duration
// of the class load to prevent concurrent redefinition of not completely
// defined classes.
// case 1. traditional classloaders that rely on the classloader object lock
// - no other need for LOAD_INSTANCE
// case 2. traditional classloaders that break the classloader object lock
// as a deadlock workaround. Detection of this case requires that
// this check is done while holding the classloader object lock,
// and that lock is still held when calling classloader's loadClass.
// For these classloaders, we ensure that the first requestor
// completes the load and other requestors wait for completion.
// case 3. UnsyncloadClass - don't use objectLocker
// With this flag, we allow parallel classloading of a
// class/classloader pair
// case4. Bootstrap classloader - don't own objectLocker
// This classloader supports parallelism at the classloader level,
// but only allows a single load of a class/classloader pair.
// No performance benefit and no deadlock issues.
// case 5. parallelCapable user level classloaders - without objectLocker
// Allow parallel classloading of a class/classloader pair

{
MutexLocker mu(SystemDictionary_lock, THREAD);
if (class_loader.is_null() || !is_parallelCapable(class_loader)) {
PlaceholderEntry* oldprobe = placeholders()->get_entry(p_index, p_hash, name, loader_data);
if (oldprobe) {
// only need check_seen_thread once, not on each loop
// 6341374 java/lang/Instrument with -Xcomp
if (oldprobe->check_seen_thread(THREAD, PlaceholderTable::LOAD_INSTANCE)) {
throw_circularity_error = true;
} else {
// case 1: traditional: should never see load_in_progress.
while (!class_has_been_loaded && oldprobe && oldprobe->instance_load_in_progress()) {

// case 4: bootstrap classloader: prevent futile classloading,
// wait on first requestor
if (class_loader.is_null()) {
SystemDictionary_lock->wait();
} else {
// case 2: traditional with broken classloader lock. wait on first
// requestor.
double_lock_wait(lockObject, THREAD);
}
// Check if classloading completed while we were waiting
Klass* check = find_class(d_index, d_hash, name, loader_data);
if (check != NULL) {
// Klass is already loaded, so just return it
k = instanceKlassHandle(THREAD, check);
class_has_been_loaded = true;
}
// check if other thread failed to load and cleaned up
oldprobe = placeholders()->get_entry(p_index, p_hash, name, loader_data);
}
}
}
}
// All cases: add LOAD_INSTANCE holding SystemDictionary_lock
// case 3: UnsyncloadClass || case 5: parallelCapable: allow competing threads to try
// LOAD_INSTANCE in parallel

if (!throw_circularity_error && !class_has_been_loaded) {
PlaceholderEntry* newprobe = placeholders()->find_and_add(p_index, p_hash, name, loader_data, PlaceholderTable::LOAD_INSTANCE, NULL, THREAD);
load_instance_added = true;
// For class loaders that do not acquire the classloader object lock,
// if they did not catch another thread holding LOAD_INSTANCE,
// need a check analogous to the acquire ObjectLocker/find_class
// i.e. now that we hold the LOAD_INSTANCE token on loading this class/CL
// one final check if the load has already completed
// class loaders holding the ObjectLock shouldn't find the class here
Klass* check = find_class(d_index, d_hash, name, loader_data);
if (check != NULL) {
// Klass is already loaded, so return it after checking/adding protection domain
k = instanceKlassHandle(THREAD, check);
class_has_been_loaded = true;
}
}
}

// must throw error outside of owning lock
if (throw_circularity_error) {
assert(!HAS_PENDING_EXCEPTION && load_instance_added == false,"circularity error cleanup");
ResourceMark rm(THREAD);
THROW_MSG_NULL(vmSymbols::java_lang_ClassCircularityError(), name->as_C_string());
}

if (!class_has_been_loaded) {

// Do actual loading
k = load_instance_class(name, class_loader, THREAD);

// For UnsyncloadClass only
// If they got a linkageError, check if a parallel class load succeeded.
// If it did, then for bytecode resolution the specification requires
// that we return the same result we did for the other thread, i.e. the
// successfully loaded InstanceKlass
// Should not get here for classloaders that support parallelism
// with the new cleaner mechanism, even with AllowParallelDefineClass
// Bootstrap goes through here to allow for an extra guarantee check
if (UnsyncloadClass || (class_loader.is_null())) {
if (k.is_null() && HAS_PENDING_EXCEPTION
&& PENDING_EXCEPTION->is_a(SystemDictionary::LinkageError_klass())) {
MutexLocker mu(SystemDictionary_lock, THREAD);
Klass* check = find_class(d_index, d_hash, name, loader_data);
if (check != NULL) {
// Klass is already loaded, so just use it
k = instanceKlassHandle(THREAD, check);
CLEAR_PENDING_EXCEPTION;
guarantee((!class_loader.is_null()), "dup definition for bootstrap loader?");
}
}
}

// If everything was OK (no exceptions, no null return value), and
// class_loader is NOT the defining loader, do a little more bookkeeping.
if (!HAS_PENDING_EXCEPTION && !k.is_null() &&
k->class_loader() != class_loader()) {

check_constraints(d_index, d_hash, k, class_loader, false, THREAD);

// Need to check for a PENDING_EXCEPTION again; check_constraints
// can throw and doesn't use the CHECK macro.
if (!HAS_PENDING_EXCEPTION) {
{ // Grabbing the Compile_lock prevents systemDictionary updates
// during compilations.
MutexLocker mu(Compile_lock, THREAD);
update_dictionary(d_index, d_hash, p_index, p_hash,
k, class_loader, THREAD);
}

if (JvmtiExport::should_post_class_load()) {
Thread *thread = THREAD;
assert(thread->is_Java_thread(), "thread->is_Java_thread()");
JvmtiExport::post_class_load((JavaThread *) thread, k());
}
}
}
} // load_instance_class loop

if (HAS_PENDING_EXCEPTION) {
// An exception, such as OOM could have happened at various places inside
// load_instance_class. We might have partially initialized a shared class
// and need to clean it up.
if (class_loader.is_null()) {
// In some cases k may be null. Let's find the shared class again.
instanceKlassHandle ik(THREAD, find_shared_class(name));
if (ik.not_null()) {
if (ik->class_loader_data() == NULL) {
// We didn't go as far as Klass::restore_unshareable_info(),
// so nothing to clean up.
} else {
Klass *kk;
{
MutexLocker mu(SystemDictionary_lock, THREAD);
kk = find_class(d_index, d_hash, name, ik->class_loader_data());
}
if (kk != NULL) {
// No clean up is needed if the shared class has been entered
// into system dictionary, as load_shared_class() won't be called
// again.
} else {
// This must be done outside of the SystemDictionary_lock to
// avoid deadlock.
//
// Note that Klass::restore_unshareable_info (called via
// load_instance_class above) is also called outside
// of SystemDictionary_lock. Other threads are blocked from
// loading this class because they are waiting on the
// SystemDictionary_lock until this thread removes
// the placeholder below.
//
// This need to be re-thought when parallel-capable non-boot
// classloaders are supported by CDS (today they're not).
clean_up_shared_class(ik, class_loader, THREAD);
}
}
}
}
}

if (load_instance_added == true) {
// clean up placeholder entries for LOAD_INSTANCE success or error
// This brackets the SystemDictionary updates for both defining
// and initiating loaders
MutexLocker mu(SystemDictionary_lock, THREAD);
placeholders()->find_and_remove(p_index, p_hash, name, loader_data, PlaceholderTable::LOAD_INSTANCE, THREAD);
SystemDictionary_lock->notify_all();
}
}

if (HAS_PENDING_EXCEPTION || k.is_null()) {
return NULL;
}

post_class_load_event(class_load_start_time, k, class_loader);

#ifdef ASSERT
{
ClassLoaderData* loader_data = k->class_loader_data();
MutexLocker mu(SystemDictionary_lock, THREAD);
Klass* kk = find_class(name, loader_data);
assert(kk == k(), "should be present in dictionary");
}
#endif

// return if the protection domain in NULL
if (protection_domain() == NULL) return k();

// Check the protection domain has the right access
{
MutexLocker mu(SystemDictionary_lock, THREAD);
// Note that we have an entry, and entries can be deleted only during GC,
// so we cannot allow GC to occur while we're holding this entry.
// We're using a No_Safepoint_Verifier to catch any place where we
// might potentially do a GC at all.
// Dictionary::do_unloading() asserts that classes in SD are only
// unloaded at a safepoint. Anonymous classes are not in SD.
No_Safepoint_Verifier nosafepoint;
if (dictionary()->is_valid_protection_domain(d_index, d_hash, name,
loader_data,
protection_domain)) {
return k();
}
}

// Verify protection domain. If it fails an exception is thrown
validate_protection_domain(k, class_loader, protection_domain, CHECK_NULL);

return k();
}

resolve_instance_class_or_null 方法的入口处可以看到如果 Class 已经被加载,则直接返回。我们也可以写个简单的单元测试来验证:

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package me.tianshuang;

public class ClassLoaderTest {

public static void main(String[] args) throws ClassNotFoundException {
ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
System.out.println(systemClassLoader.loadClass("java.lang.Boolean") == systemClassLoader.loadClass("java.lang.Boolean"));
System.out.println(Class.forName("java.lang.Integer") == Class.forName("java.lang.Integer"));
}

}

运行后输出均为 true。可以确认使用同一个类加载器实例去加载同名的类,只会对该类加载一次。

再看看 com.alibaba.compileflow.engine.process.preruntime.compiler.impl.FlowClassLoader 类加载器的实现:

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/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.alibaba.compileflow.engine.process.preruntime.compiler.impl;

import com.alibaba.compileflow.engine.common.CompileFlowException;
import com.alibaba.compileflow.engine.process.preruntime.compiler.CompileConstants;

import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

/**
* @author yusu
*/
public class FlowClassLoader extends URLClassLoader {

// 使用 volatile 修饰保证可见性
private static volatile FlowClassLoader instance = null;

// 此处可以优化为 private
public FlowClassLoader(URL[] urls, ClassLoader parent) {
super(urls, parent);
}

// 通过双重检查锁实现单例模式
public static FlowClassLoader getInstance() {
if (instance == null) {
synchronized (FlowClassLoader.class) {
if (instance == null) {
try {
URL url = new URL("file:///" + CompileConstants.FLOW_COMPILE_CLASS_DIR);
instance = new FlowClassLoader(new URL[] {url},
FlowClassLoader.class.getClassLoader());
} catch (MalformedURLException e) {
throw new CompileFlowException(e);
}
}
}
}
return instance;
}

// 将引用至空以保证后续新建类加载器去加载类
public void clearCache() {
instance = null;
}

public Class<?> defineClass(String name, byte[] classBytes) {
return defineClass(name, classBytes, 0, classBytes.length);
}

}

我们编写如下的单元测试尝试用不同的字节流对同一个 name 进行 defineClass 以尝试获取不同的 class 实例:

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@Test
public void test() throws IOException, InstantiationException, IllegalAccessException {
FlowClassLoader flowClassLoader = FlowClassLoader.getInstance();

Class<?> first = flowClassLoader.defineClass("Test", FileUtils.readFileToByteArray(new File("/Users/Poison/IdeaProjects/compileflow/.flowclasses/compileflow/first.class")));
Class<?> second = flowClassLoader.defineClass("Test", FileUtils.readFileToByteArray(new File("/Users/Poison/IdeaProjects/compileflow/.flowclasses/compileflow/second.class")));

first.newInstance();
second.newInstance();
}

运行时直接抛出如下异常:

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java.lang.LinkageError: loader (instance of  com/alibaba/compileflow/engine/process/preruntime/compiler/impl/FlowClassLoader): attempted  duplicate class definition for name: "Test"

at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:756)
at java.lang.ClassLoader.defineClass(ClassLoader.java:635)
at com.alibaba.compileflow.engine.process.preruntime.compiler.impl.FlowClassLoader.defineClass(FlowClassLoader.java:59)
at com.allibaba.compileflow.test.mock.TestRunTime.test(TestRunTime.java:36)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)

可见 defineClass 不允许使用不同的字节流定义同名的类,底层具体的执行逻辑读者可以查看 OpenJDK 中相关源码,此处不再分析相关代码。

根据以上的逻辑,就不难理解在 compileflow 中重新加载类的实现,其采用重新创建一个类加载器去定义类的方式以实现类的重新加载,相关源码位于 AbstractProcessEngine#reload:

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@Override
public void reload(String code) {
// 此处将单例的 instance 置空,下次获取 FlowClassLoader 的实例时会重新创建类加载器实例
FlowClassLoader.getInstance().clearCache();
AbstractProcessRuntime runtime = runtimeCache.computeIfPresent(code, (k, v) -> getRuntimeFromSource(code));
runtime.recompile(code);
}

重新加载动态生成类的代码位于 CompilerImpl:

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@Override
public Class<?> compileJavaCode(String fullClassName, String sourceCode) {

try {
String dirPath = CompileConstants.FLOW_COMPILE_CLASS_DIR;
File dirFile = new File(dirPath);
if (!dirFile.exists()) {
if (!dirFile.mkdir()) {
throw new RuntimeException(
"Output directory for process class can't be created, directory is " + dirPath);
}
}

File javaSourceFile = writeJavaFile(dirFile, fullClassName, sourceCode);
JavaSource javaSource = JavaSource.of(javaSourceFile, sourceCode, fullClassName);

JAVA_COMPILER.compile(javaSource, new File(dirPath), new CompileOption());

File classFile = new File(dirFile, fullClassName.replace('.', File.separatorChar) + ".class");
byte[] classBytes = FileUtils.readFileToByteArray(classFile);
return FlowClassLoader.getInstance().defineClass(fullClassName, classBytes);
} catch (CompileFlowException e) {
throw e;
} catch (Exception e) {
throw new CompileFlowException(e.getMessage(), e);
}
}

根据以上流程,就实现了运行时动态加载类的功能。

Reference

How to replace classes in a running application in java ?
Dynamically recompile and reload a class
Multiple loading of a class thru Class.forName() ??
Singleton Design Pattern in Java