類載入機制，雙親委派的那些事
阿新 • • 發佈：2020-07-22
首先我們來看張圖，自己就不畫了，網上找了張圖。
（記憶體模型就先不看了，1。8之後已經不是這個模型了）
我們java的程式在載入到jvm之前，經歷過的事情上面在類載入器之前我就不解釋了，在.class檔案到類載入器直接，還有一些過程。看下圖
載入：io讀取磁碟上面的位元組碼檔案，在物件被使用到的時候載入到記憶體
驗證：驗證記憶體中讀取位元組碼檔案內容是否正確（裡面是很多0-9和字母組成，class檔案不要隨意去更改）
準備：這個階段給靜態成員分配地址，給預設值（基本型別都有預設值，如果成員中有物件的話，此時為null，即使是你程式碼中new了）
解析：符號（類名，方法名，包名）這個靜態連結過程怎麼去解釋呢，（這個理解了往後補充）
初始化：把靜態變數賦值為我們給的值
我們再看類載入器：
1：啟動類載入器：載入jre的lib下的核心庫類（rt.jar，charsets.jar等）
2：拓展類載入器：在家jre的lib下的ext拓展檔案中的jar包類
3：應用程式類載入器：載入classpath下的類（自己的類）
4：自定義類載入器：載入使用者指定的類
先來看下自定義載入器，我們如何去定義一個類載入器：
public class Test { public void helle () { System.out.println("helle_classloader"); } }
定義一個Test類
package com.ghsy.user.test; import java.io.ByteArrayOutputStream; import java.io.File; import java.io.FileInputStream; import java.io.IOException; import java.lang.reflect.InvocationTargetException; public class MyClassLoader extends ClassLoader { private String classPath;public MyClassLoader(String classPath) { this.classPath = classPath; } /** * 重寫findClass方法 * @param name 是我們這個類的全路徑 * @return * @throws ClassNotFoundException */ @Override protected Class<?> findClass(String name) throws ClassNotFoundException { Class log = null; // 獲取該class檔案位元組碼陣列 byte[] classData = getData(); if (classData != null) { // 將class的位元組碼陣列轉換成Class類的例項 log = defineClass(name, classData, 0, classData.length); } return log; } /** * 將class檔案轉化為位元組碼陣列 * @return */ private byte[] getData() { File file = new File(classPath); if (file.exists()){ FileInputStream in = null; ByteArrayOutputStream out = null; try { in = new FileInputStream(file); out = new ByteArrayOutputStream(); byte[] buffer = new byte[1024]; int size = 0; while ((size = in.read(buffer)) != -1) { out.write(buffer, 0, size); } } catch (IOException e) { e.printStackTrace(); } finally { try { in.close(); } catch (IOException e) { e.printStackTrace(); } } return out.toByteArray(); }else{ return null; } } public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException { MyClassLoader classLoader = new MyClassLoader("C:/test/Test.class"); Class clazz = classLoader.findClass("com.ghsy.user.test.Test"); Object object = clazz.newInstance(); clazz.getMethod("helle",null).invoke(object, null); System.out.println(clazz.getClassLoader().getClass().getName()); } }
自定義一個類載入器MyClassLoader。
我們把target下classes裡面的Test.class拷貝到C盤test目錄中
控制檯輸出：
D:\work\java\jdk1.8.0_60\bin\java.exe "-javaagent:D:\work\idea\IntelliJ IDEA 2018.2.5\lib\idea_rt.jar=53557:D:\work\idea\IntelliJ IDEA 2018.2.5\bin" -Dfile.encoding=UTF-8 -classpath 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che\tomcat\embed\tomcat-embed-websocket\9.0.27\tomcat-embed-websocket-9.0.27.jar;D:\work\jar-resource\org\springframework\boot\spring-boot-starter-validation\2.2.1.RELEASE\spring-boot-starter-validation-2.2.1.RELEASE.jar;D:\work\jar-resource\jakarta\validation\jakarta.validation-api\2.0.1\jakarta.validation-api-2.0.1.jar;D:\work\jar-resource\org\hibernate\validator\hibernate-validator\6.0.18.Final\hibernate-validator-6.0.18.Final.jar;D:\work\jar-resource\org\jboss\logging\jboss-logging\3.4.1.Final\jboss-logging-3.4.1.Final.jar;D:\work\jar-resource\com\fasterxml\classmate\1.5.1\classmate-1.5.1.jar;D:\work\jar-resource\org\springframework\spring-web\5.2.1.RELEASE\spring-web-5.2.1.RELEASE.jar;D:\work\jar-resource\org\springframework\spring-beans\5.2.1.RELEASE\spring-beans-5.2.1.RELEASE.jar;D:\work\jar-resource\org\springframework\spring-webmvc\5.2.1.RELEASE\spring-webmvc-5.2.1.RELEASE.jar;D:\work\jar-resource\org\springframework\spring-aop\5.2.1.RELEASE\spring-aop-5.2.1.RELEASE.jar;D:\work\jar-resource\org\springframework\spring-expression\5.2.1.RELEASE\spring-expression-5.2.1.RELEASE.jar com.ghsy.user.test.MyClassLoader
helle_classloader
com.ghsy.user.test.MyClassLoader
Process finished with exit code 0
這個時候ClassLoader是我們自己定義的ClassLoader，不再是由AppClassLoader去載入了。
問題1：如果自定義一個類載入器去載入？
繼承ClassLoader，重寫裡面的findClass方法。
在看下這些類載入器之間的關係：
自己畫的一個草圖，上面的null，網上由其他資料說是BootstrapClassLoader，我在jdk中並未找到明確的類（無法作證），這個啟動類載入器為低層c語言實現（navtice），不需要去特意關注。
這裡提一個經常提到的概念：類載入器的雙親委派機制，下面我們來看原始碼（ClassLoader中的loadclass方法）
我們先來求證一件事情，上圖所對應的類載入器關係是不是這樣呢？我們看AppClassLoader的原始碼：
static class AppClassLoader extends URLClassLoader { final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
//這個方法返回的是一個AppClassLoader public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException { //傳入的為ClassLoader實現類(這裡為ExtClassLoader，下面會解釋) final String var1 = System.getProperty("java.class.path"); final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1); return (ClassLoader)AccessController.doPrivileged(new PrivilegedAction<Launcher.AppClassLoader>() { public Launcher.AppClassLoader run() { URL[] var1x = var1 == null ? new URL[0] : Launcher.pathToURLs(var2); return new Launcher.AppClassLoader(var1x, var0); } }); } AppClassLoader(URL[] var1, ClassLoader var2) { super(var1, var2, Launcher.factory); //呼叫父類的建構函式（看第三部分程式碼） this.ucp.initLookupCache(this); } public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException { int var3 = var1.lastIndexOf(46); if (var3 != -1) { SecurityManager var4 = System.getSecurityManager(); if (var4 != null) { var4.checkPackageAccess(var1.substring(0, var3)); } } if (this.ucp.knownToNotExist(var1)) { Class var5 = this.findLoadedClass(var1); if (var5 != null) { if (var2) { this.resolveClass(var5); } return var5; } else { throw new ClassNotFoundException(var1); } } else { return super.loadClass(var1, var2); } } protected PermissionCollection getPermissions(CodeSource var1) { PermissionCollection var2 = super.getPermissions(var1); var2.add(new RuntimePermission("exitVM")); return var2; } private void appendToClassPathForInstrumentation(String var1) { assert Thread.holdsLock(this); super.addURL(Launcher.getFileURL(new File(var1))); } private static AccessControlContext getContext(File[] var0) throws MalformedURLException { PathPermissions var1 = new PathPermissions(var0); ProtectionDomain var2 = new ProtectionDomain(new CodeSource(var1.getCodeBase(), (Certificate[])null), var1); AccessControlContext var3 = new AccessControlContext(new ProtectionDomain[]{var2}); return var3; } static { ClassLoader.registerAsParallelCapable(); } }
//這裡為LaunCh的構造方法原始碼(AppClassLoader和ExtClassLoader都為LaunCh類的內部類)
public Launcher() {
Launcher.ExtClassLoader var1;
try {
var1 = Launcher.ExtClassLoader.getExtClassLoader(); //獲取了一個ExtClassLoader
} catch (IOException var10) {
throw new InternalError("Could not create extension class loader", var10);
}
try {
this.loader = Launcher.AppClassLoader.getAppClassLoader(var1); //傳入的是ExtClassLoader（由此證明上面AppClassLoader的getAppClassLoader方法傳入的是ExtClassLoader）
} catch (IOException var9) {
throw new InternalError("Could not create application class loader", var9);
}
Thread.currentThread().setContextClassLoader(this.loader);
String var2 = System.getProperty("java.security.manager");
if (var2 != null) {
SecurityManager var3 = null;
if (!"".equals(var2) && !"default".equals(var2)) {
try {
var3 = (SecurityManager)this.loader.loadClass(var2).newInstance();
} catch (IllegalAccessException var5) {
;
} catch (InstantiationException var6) {
;
} catch (ClassNotFoundException var7) {
;
} catch (ClassCastException var8) {
;
}
} else {
var3 = new SecurityManager();
}
if (var3 == null) {
throw new InternalError("Could not create SecurityManager: " + var2);
}
System.setSecurityManager(var3);
}
}
//URLClassLoader（AppClassLoader和ExtClassLoader的父類）的構造方法 在點進原始碼中去看，最終發現URLClassLoader是ClassLoader子類
public URLClassLoader(URL[] urls, ClassLoader parent,
URLStreamHandlerFactory factory) {
super(parent);
// this is to make the stack depth consistent with 1.1
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
ucp = new URLClassPath(urls, factory);
acc = AccessController.getContext();
}
//SecureClassLoader（URLClassLoader父類）中的構造方法，繼續呼叫父類的構造方法，我們再去ClassLoader中看
protected SecureClassLoader(ClassLoader parent) {
super(parent);
// this is to make the stack depth consistent with 1.1
SecurityManager security = System.getSecurityManager();
if (security != null) {
security.checkCreateClassLoader();
}
initialized = true;
}
//ClassLoader
private ClassLoader(Void unused, ClassLoader parent) { //這裡可以看到，AppClassLoader的Parent其實已經是個ExtClssLoader的例項，
//同理，ExtClassLoader都為URLClassLoader子類，這裡就做過多贅述
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;
}
}
//再來看下ExtClassLoader的建構函式和getExtDirs方法，由此可證(ExtClassLoader載入的是ext擴充套件包中的類)
public ExtClassLoader(File[] var1) throws IOException {
super(getExtURLs(var1), (ClassLoader)null, Launcher.factory); //建構函式傳入的ClassLoader是null，說明一開始執行的main函式輸出String的類載入器為null是正確的，
//網上其他BootStrapClassLoader人云亦云
SharedSecrets.getJavaNetAccess().getURLClassPath(this).initLookupCache(this);
}
private static File[] getExtDirs() {
String var0 = System.getProperty("java.ext.dirs");
File[] var1;
if (var0 != null) {
StringTokenizer var2 = new StringTokenizer(var0, File.pathSeparator);
int var3 = var2.countTokens();
var1 = new File[var3];
for(int var4 = 0; var4 < var3; ++var4) {
var1[var4] = new File(var2.nextToken());
}
} else {
var1 = new File[0];
}
return var1;
}
//最後我們來看下loadclass方發，什麼情況下去呼叫的啟動類載入器(null)
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); //這個位置是ExtClassLoader載入器所進入到的判斷（具體為啥往上翻，因為ExtClassLoader建構函式裡面parent傳入的就是null）
}
} 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;
}
}
//再來看findBootstrapClassOrNull方法
/**
* Returns a class loaded by the bootstrap class loader; //返回由啟動類載入器在載的Class，如果啟動類無法載入，則返回null
* or return null if not found.
*/
private Class<?> findBootstrapClassOrNull(String name)
{
if (!checkName(name)) return null;
return findBootstrapClass(name);
}
//findBootstrapClass方法為native方法（由c語言實現，不可見）次方法為載入jdk核心jar類的方法
// return null if not found
private native Class<?> findBootstrapClass(String name);
問題1：自定義類載入器，為什麼預設的parent會是AppClassLoader呢？我們來看ClassLoader的預設建構函式：
//預設建構函式 protected ClassLoader() { this(checkCreateClassLoader(), getSystemClassLoader()); } //帶有引數的建構函式，上面預設的建構函式呼叫了可以看到，這時候已經傳入了parent，我們再來看getSystemClassLoader方法 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; } }
//getSystemClassLoader方法，呼叫了initSystemClassLoader方法，並且返回了scl(內部變數ClassLoader)，接著來看initSystemClassLoader方法
public static ClassLoader getSystemClassLoader() {
initSystemClassLoader();
if (scl == null) {
return null;
}
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkClassLoaderPermission(scl, Reflection.getCallerClass());
}
return scl;
}
//initSystemClassLoader方法
private static synchronized void initSystemClassLoader() {
if (!sclSet) {
if (scl != null)
throw new IllegalStateException("recursive invocation");
sun.misc.Launcher l = sun.misc.Launcher.getLauncher(); //返回了Launcher物件(包含了內部類AppClassLoader和ExtClassLoader)，這個方法不過多解釋，看一下就知道返回了一個LaunCher內部變數LaunCher
if (l != null) {
Throwable oops = null;
scl = l.getClassLoader(); //我們再看這個getClassLoader方法，返回了內部成員CLassLoader，這個是AppClassLoader（具體為什麼請看上面的Launcher類的建構函式，原始碼第49行）
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;
}
}
至此，預設的parent賦值為AppClassLoader，上面畫的個個類載入器關係全部串聯起來！！！！！！
答案在上面程式碼中，因為預設的建構函式給parent初始化為AppClassLoader了。
問題2：一個類的載入過程是怎麼樣的？
先去找自定義類的載入器是否