执行java main方法，先得加载main方法的class，也就是java.c的LoadMainClass方法加载class。
//jdk /src/share/bin/java.c
/*
* Loads a class and verifies that the main class is present and it is ok to
* call it for more details refer to the java implementation.
*/
static jclass
LoadMainClass(JNIEnv *env, int mode, char *name)
{
jmethodID mid;
jstring str;
jobject result;
jlong start, end;
jclass cls = GetLauncherHelperClass(env)</red>;
NULL_CHECK0(cls);
if (JLI_IsTraceLauncher()) {
start = CounterGet();
}
NULL_CHECK0(mid = (*env)->GetStaticMethodID(env, cls,
"checkAndLoadMain",
"(ZILjava/lang/String;)Ljava/lang/Class;"));
switch (mode) {
case LM_CLASS:
str = NewPlatformString(env, name);
break;
default:
str = (*env)->NewStringUTF(env, name);
break;
}
result = (*env)->CallStaticObjectMethod(env, cls, mid, USE_STDERR, mode, str);
if (JLI_IsTraceLauncher()) {
end = CounterGet();
printf("%ld micro seconds to load main class\n",
(long)(jint)Counter2Micros(end-start));
printf("----_JAVA_LAUNCHER_DEBUG----\n");
}
return (jclass)result;
}
上面jdk代码java.c的LoadMainClass方法调用了java代码包rt.jar的sun.launcher.LauncherHelper.java的checkAndLoadMain方法，而LauncherHelper.java的checkAndLoadMain方法通过调java.lang.ClassLoader.loadClass()加载class。
//sun.launcher.LauncherHelper
/**
* This method does the following:
* 1. gets the classname from a Jar's manifest, if necessary
* 2. loads the class using the System ClassLoader
* 3. ensures the availability and accessibility of the main method,
* using signatureDiagnostic method.
* a. does the class exist
* b. is there a main
* c. is the main public
* d. is the main static
* c. does the main take a String array for args
* 4. and off we go......
*
* @param printToStderr
* @param isJar
* @param name
* @return
*/
public static Class<?> checkAndLoadMain(boolean printToStderr,
int mode,
String what) {
final PrintStream ostream = (printToStderr) ? System.err : System.out;
final ClassLoader ld = ClassLoader.getSystemClassLoader();
// get the class name
String cn = null;
switch (mode) {
case LM_CLASS:
cn = what;
break;
case LM_JAR:
cn = getMainClassFromJar(ostream, what);
break;
default:
// should never happen
throw new InternalError("" + mode + ": Unknown launch mode");
}
cn = cn.replace('/', '.');
Class<?> c = null;
try {
c = ld.loadClass(cn);
} catch (ClassNotFoundException cnfe) {
abort(ostream, cnfe, "java.launcher.cls.error1", cn);
}
getMainMethod(ostream, c);
return c;
}
我们都知道加载class是java的java.lang.ClassLoader根据双亲委派加载机制加载的，也就是ClassLoader.loadClass()方法，这里定义了双亲委派加载逻辑。
//java.lang.ClassLoader
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;
}
}
findLoadedClass查找已经装载的class，findBootstrapClassOrNull根加载器加载查找，如果class未加载，父类加载器也加载不了，则用子类加载器加载。
子类加载器的findClass方法都是实现类URLClassLoader的findClass方法，因为我们的加载器是AppClassLoader，继承了URLClassLoader。
//java.net.URLClassLoader
protected Class<?> findClass(final String name)
throws ClassNotFoundException
{
try {
return AccessController.doPrivileged(
new PrivilegedExceptionAction<Class>() {
public Class run() throws ClassNotFoundException {
String path = name.replace('.', '/').concat(".class");
Resource res = ucp.getResource(path, false);
if (res != null) {
try {
return defineClass(name, res);
} catch (IOException e) {
throw new ClassNotFoundException(name, e);
}
} else {
throw new ClassNotFoundException(name);
}
}
}, acc);
} catch (java.security.PrivilegedActionException pae) {
throw (ClassNotFoundException) pae.getException();
}
}
private Class defineClass(String name, Resource res) throws IOException {
long t0 = System.nanoTime();
int i = name.lastIndexOf('.');
URL url = res.getCodeSourceURL();
if (i != -1) {
String pkgname = name.substring(0, i);
// Check if package already loaded.
Manifest man = res.getManifest();
if (getAndVerifyPackage(pkgname, man, url) == null) {
try {
if (man != null) {
definePackage(pkgname, man, url);
} else {
definePackage(pkgname, null, null, null, null, null, null, null);
}
} catch (IllegalArgumentException iae) {
// parallel-capable class loaders: re-verify in case of a
// race condition
if (getAndVerifyPackage(pkgname, man, url) == null) {
// Should never happen
throw new AssertionError("Cannot find package " +
pkgname);
}
}
}
}
// Now read the class bytes and define the class
java.nio.ByteBuffer bb = res.getByteBuffer();
if (bb != null) {
// Use (direct) ByteBuffer:
CodeSigner[] signers = res.getCodeSigners();
CodeSource cs = new CodeSource(url, signers);
sun.misc.PerfCounter.getReadClassBytesTime().addElapsedTimeFrom(t0);
return defineClass(name, bb, cs);
} else {
byte[] b = res.getBytes();
// must read certificates AFTER reading bytes.
CodeSigner[] signers = res.getCodeSigners();
CodeSource cs = new CodeSource(url, signers);
sun.misc.PerfCounter.getReadClassBytesTime().addElapsedTimeFrom(t0);
return defineClass(name, b, 0, b.length, cs);
}
}
URLClassLoader的defineClass方法调用的是父类SecureClassLoaderdefineClass方法，defineClass继续调defineClass1这个是native方法。
protected final Class<?> defineClass(String name, java.nio.ByteBuffer b,
CodeSource cs)
{
return defineClass(name, b, getProtectionDomain(cs));
}
protected final Class<?> defineClass(String name, byte[] b, int off, int len,
ProtectionDomain protectionDomain)
throws ClassFormatError
{
protectionDomain = preDefineClass(name, protectionDomain);
Class c = null;
String source = defineClassSourceLocation(protectionDomain);
try {
c = defineClass1(name, b, off, len, protectionDomain, source);
} catch (ClassFormatError cfe) {
c = defineTransformedClass(name, b, off, len, protectionDomain, cfe,
source);
}
postDefineClass(c, protectionDomain);
return c;
}
private native Class defineClass1(String name, byte[] b, int off, int len,
ProtectionDomain pd, String source);
我们再看jdk里面的native实现，之前说过native代码都在/src/share/native下，java通过JNI调到jdk的c代码。
// jdk /src/share/native/java/lang/ClassLoader.c
JNIEXPORT jclass JNICALL
Java_java_lang_ClassLoader_defineClass1(JNIEnv *env,
jobject loader,
jstring name,
jbyteArray data,
jint offset,
jint length,
jobject pd,
jstring source)
{
jbyte *body;
char *utfName;
jclass result = 0;
char buf[128];
char* utfSource;
char sourceBuf[1024];
if (data == NULL) {
JNU_ThrowNullPointerException(env, 0);
return 0;
}
/* Work around 4153825. malloc crashes on Solaris when passed a
* negative size.
*/
if (length < 0) {
JNU_ThrowArrayIndexOutOfBoundsException(env, 0);
return 0;
}
body = (jbyte *)malloc(length);
if (body == 0) {
JNU_ThrowOutOfMemoryError(env, 0);
return 0;
}
(*env)->GetByteArrayRegion(env, data, offset, length, body);
if ((*env)->ExceptionOccurred(env))
goto free_body;
if (name != NULL) {
utfName = getUTF(env, name, buf, sizeof(buf));
if (utfName == NULL) {
JNU_ThrowOutOfMemoryError(env, NULL);
goto free_body;
}
VerifyFixClassname(utfName);
} else {
utfName = NULL;
}
if (source != NULL) {
utfSource = getUTF(env, source, sourceBuf, sizeof(sourceBuf));
if (utfSource == NULL) {
JNU_ThrowOutOfMemoryError(env, NULL);
goto free_utfName;
}
} else {
utfSource = NULL;
}
result = JVM_DefineClassWithSource(env, utfName, loader, body, length, pd, utfSource);
if (utfSource && utfSource != sourceBuf)
free(utfSource);
free_utfName:
if (utfName && utfName != buf)
free(utfName);
free_body:
free(body);
return result;
}
而ClassLoader.c调用的JVM_DefineClassWithSource方法调用到的是JVM代码，一般JVM_开头的方法都是调JVM的，是引入与的jvm.h中定义的。
JVM_ENTRY(jclass, JVM_DefineClassWithSource(JNIEnv *env, const char *name, jobject loader, const jbyte *buf, jsize len, jobject pd, const char *source))
JVMWrapper2("JVM_DefineClassWithSource %s", name);
return jvm_define_class_common(env, name, loader, buf, len, pd, source, true, THREAD);
JVM_END
// common code for JVM_DefineClass() and JVM_DefineClassWithSource()
// and JVM_DefineClassWithSourceCond()
static jclass jvm_define_class_common(JNIEnv *env, const char *name,
jobject loader, const jbyte *buf,
jsize len, jobject pd, const char *source,
jboolean verify, TRAPS) {
if (source == NULL) source = "__JVM_DefineClass__";
assert(THREAD->is_Java_thread(), "must be a JavaThread");
JavaThread* jt = (JavaThread*) THREAD;
PerfClassTraceTime vmtimer(ClassLoader::perf_define_appclass_time(),
ClassLoader::perf_define_appclass_selftime(),
ClassLoader::perf_define_appclasses(),
jt->get_thread_stat()->perf_recursion_counts_addr(),
jt->get_thread_stat()->perf_timers_addr(),
PerfClassTraceTime::DEFINE_CLASS);
if (UsePerfData) {
ClassLoader::perf_app_classfile_bytes_read()->inc(len);
}
// Since exceptions can be thrown, class initialization can take place
// if name is NULL no check for class name in .class stream has to be made.
TempNewSymbol class_name = NULL;
if (name != NULL) {
const int str_len = (int)strlen(name);
if (str_len > Symbol::max_length()) {
// It's impossible to create this class; the name cannot fit
// into the constant pool.
THROW_MSG_0(vmSymbols::java_lang_NoClassDefFoundError(), name);
}
class_name = SymbolTable::new_symbol(name, str_len, CHECK_NULL);
}
ResourceMark rm(THREAD);
ClassFileStream st((u1*) buf, len, (char *)source);
Handle class_loader (THREAD, JNIHandles::resolve(loader));
if (UsePerfData) {
is_lock_held_by_thread(class_loader,
ClassLoader::sync_JVMDefineClassLockFreeCounter(),
THREAD);
}
Handle protection_domain (THREAD, JNIHandles::resolve(pd));
Klass* k = SystemDictionary::resolve_from_stream(class_name, class_loader,
protection_domain, &st,
verify != 0,
CHECK_NULL);
if (TraceClassResolution && k != NULL) {
trace_class_resolution(k);
}
return (jclass) JNIHandles::make_local(env, k->java_mirror());
}
最重要的一个方法调用是SystemDictionary::resolve_from_stream方法，SystemDictionary是字典Dictionary一个工具类，SystemDictionary解析完的class放在
Dictionary中，查找已加载的class也是从这个字典中hash查找。
// Add a klass to the system from a stream (called by jni_DefineClass and
// JVM_DefineClass).
// Note: class_name can be NULL. In that case we do not know the name of
// the class until we have parsed the stream.
Klass* SystemDictionary::resolve_from_stream(Symbol* class_name,
Handle class_loader,
Handle protection_domain,
ClassFileStream* st,
bool verify,
TRAPS) {
// Classloaders that support parallelism, e.g. bootstrap classloader,
// or all classloaders with UnsyncloadClass do not acquire lock here
bool DoObjectLock = true;
if (is_parallelCapable(class_loader)) {
DoObjectLock = false;
}
ClassLoaderData* loader_data = register_loader(class_loader, CHECK_NULL);
// 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);
TempNewSymbol parsed_name = NULL;
// Parse the stream. Note that we do this even though this klass might
// already be present in the SystemDictionary, otherwise we would not
// throw potential ClassFormatErrors.
//
// Note: "name" is updated.
instanceKlassHandle k = ClassFileParser(st).parseClassFile(class_name,
loader_data,
protection_domain,
parsed_name,
verify,
THREAD);
const char* pkg = "java/";
if (!HAS_PENDING_EXCEPTION &&
!class_loader.is_null() &&
parsed_name != NULL &&
!strncmp((const char*)parsed_name->bytes(), pkg, strlen(pkg))) {
// It is illegal to define classes in the "java." package from
// JVM_DefineClass or jni_DefineClass unless you're the bootclassloader
ResourceMark rm(THREAD);
char* name = parsed_name->as_C_string();
char* index = strrchr(name, '/');
*index = '\0'; // chop to just the package name
while ((index = strchr(name, '/')) != NULL) {
*index = '.'; // replace '/' with '.' in package name
}
const char* fmt = "Prohibited package name: %s";
size_t len = strlen(fmt) + strlen(name);
char* message = NEW_RESOURCE_ARRAY(char, len);
jio_snprintf(message, len, fmt, name);
Exceptions::_throw_msg(THREAD_AND_LOCATION,
vmSymbols::java_lang_SecurityException(), message);
}
if (!HAS_PENDING_EXCEPTION) {
assert(parsed_name != NULL, "Sanity");
assert(class_name == NULL || class_name == parsed_name, "name mismatch");
// Verification prevents us from creating names with dots in them, this
// asserts that that's the case.
assert(is_internal_format(parsed_name),
"external class name format used internally");
// Add class just loaded
// If a class loader supports parallel classloading handle parallel define requests
// find_or_define_instance_class may return a different InstanceKlass
if (is_parallelCapable(class_loader)) {
k = find_or_define_instance_class(class_name, class_loader, k, THREAD);
} else {
define_instance_class(k, THREAD);
}
}
// Make sure we have an entry in the SystemDictionary on success
debug_only( {
if (!HAS_PENDING_EXCEPTION) {
assert(parsed_name != NULL, "parsed_name is still null?");
Symbol* h_name = k->name();
ClassLoaderData *defining_loader_data = k->class_loader_data();
MutexLocker mu(SystemDictionary_lock, THREAD);
Klass* check = find_class(parsed_name, loader_data);
assert(check == k(), "should be present in the dictionary");
Klass* check2 = find_class(h_name, defining_loader_data);
assert(check == check2, "name inconsistancy in SystemDictionary");
}
} );
return k();
}
因此最终加载class是JVM中SystemDictionary.cpp的resolve_from_stream加载class，这里面会解析class，封装成Klass对象，存储起来，以便查找已加载的class。这个解析和存储的我们下一章再细讲。
总结，class通过双亲委派机制加载，如果class未加载（查SystemDictionary的Dictionary字典），调用父加载器加载，父加载器加载不了则用子加载器加载，加载完成放入到Dictionary字典中。其中加载器加载class文件通过ClassFileParser解析，解析完成的class中注解、属性、方法等放在数据结构Klass。
整个调用过程是先通过jdk的c代码java.c调用loadMainClss()方法一直调到java代码java.lang.ClassLoader.loadClass()，这个最后有调到jdk代码java/lang/ClassLoader.c，C调Java和Java调C都是JNI方式。jdk的ClassLoader.c又会调JVM的SystemDictionary加载解析class。(我们的java代码jdk也有，这里区分开主要是为了和C C++代码区分）
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