java类加载

类的生命周期(类加载过程)

LLIUU+VPR

加载(Loading)

链接(Linking)

验证(Verification)

准备(Preparation)

解析(Resolution)

初始化(Initialization)

使用(Using)

卸载(Unloading) 类

类加载器种类

BootstrapClassLoader：C++编写，负责加载java核心类库

Launcher.ExtClassLoader：Launcher中的内部类，parent == null

Launcher.AppClassLoader：Launcher中的内部类，parent == ExtClassLoader

用户自定义ClassLoader：继承自ClassLoader，parent == AppClassLoader

类加载机制

java中默认的类加载机制是双亲委派模式。

ClassLoader中关键的方法说明：

loadClass // 类加载入口，包含下面这些步骤

=> findLoadedClass => findLoadedClass0 // 先从缓存中查询一下，看看目标类是否已加载过

=> findBootstrapClassOrNull => findBootstrapClass // 用Bootstrap类加载器进行加载

=> findClass // 读取字节码文件，然后加载字节码文件

=> defineClass // 加载字节码文件

=> preDefineClass // 加载前的检查

=> defineClassSourceLocation // 定义类加载的路径

=> defineClass1/defineClass2 // 调用native方法加载类

=> postDefineClass //

=> resolveClass => resolveClass0

ClassLoader 部分源码：

package java.lang;

import java.io.InputStream;

...

public abstract class ClassLoader {

private final ClassLoader parent;

// -- Class --

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); // 缓存机制

if (c == null) {

long t0 = System.nanoTime();

try {

// 双亲委派机制

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;

}

}

// findClass由子类去实现

protected Class> findClass(String name) throws ClassNotFoundException {

throw new ClassNotFoundException(name);

}

// defineClass 加载类

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;

}

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);

}

}

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);

protected final void resolveClass(Class> c) {

resolveClass0(c);

}

private native void resolveClass0(Class> c);

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);

// -- Resource --

...

// -- Hierarchy --

...

// -- Package --

...

// -- Native library access --

...

// -- Assertion management --

...

}

双亲委派