java classloader

認識ClassLoader

林政儀 2018/09/03 10:58:28

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認識ClassLoader

簡介	認識java最基礎class載入運行原理
作者	林政儀

JVM預設ClassLoader模型

Class與ClassLoader基本特點:

jvm啟動後會建立三個基礎ClassLoader:
(a) BootStrapClassLoader: 第一個建立的ClassLoader, 負責載入jre相關classes、bin/*jar
*可由系統參數"sun.boot.class.path"設定或取得
(b) ExtClassLoader: 第二建立的ClassLoader, 負責載入jer的lib/ext/*.jar
*可由系統參數"sun.boot.class.path"設定或取得
(c) AppClassLoader: 第三建立的ClassLoader, 負責載入application相關
*可由系統參數"java.class.path"設定或取得, 或透過-cp設定
每個ClassLoader都有parentClassLoader, 該parentClassLoader並非繼承關係, 而是邏輯關係 (下面將可透過source code了解)
若ClassLoader.getParent()返回null, 看起來像是沒有parentClassLoader, 但其實是程式邏輯上控制由"BootStrapClassLoader"擔任該CLassLoader之parentClassLoader
若同個.class檔案由不同ClassLoader載入, 將視為不同的Class<?>, 意即若將ClassLoader(A)載入之test.class實例派給ClassLoader(B)的ClassType將會拋出java.lang.ClassCastException
ClassLoader最重要的兩個載入class方法:
loadClass(): 最基礎載入方法, 內部邏輯為優先調用parentClassLoader的loadClass(), 若載不到class才會呼叫findClass()
findClass(): 為protected方法, 繼承ClassLoader基本上都會Override此方法, 實作載入class方法
預設程式運作時將會使用Thread.currentThread().getContextClassLoader()的ClassLoader來載入class,
因此透過Thread.currentThread().setContextClassLoader(...), 將可抽換預設的ClassLoader,
而由此Thread fork 的其他Thread將會沿用此ContextClassLoader。
ClassLoader被GC條件為由此ClassLoader載入的所有class實例已被GC，以及相關static field不再被其他程式參照。

測試程式

AeryPractice_ClassLoader.java

package org.aery.practice.classloader;

import java.io.IOException;

import java.net.URL;

import java.net.URLClassLoader;

public class AeryPractice_ClassLoader {

public static void main(String[] args)

throws ClassNotFoundException, IOException, InstantiationException, IllegalAccessException {

printInfo();

testDifferentClassLoader();

}

/** 測試由不同ClassLoder載入相同.class之狀況 */

private static void testDifferentClassLoader()

throws ClassNotFoundException, IOException, InstantiationException, IllegalAccessException {

URL[] urls = new URL[] { //

new URL("file:/C:\\AeryLocal\\Work\\ws-aery\\test\\bin\\") //

};

String loadClassQualifiedName = "org.aery.practice.classloader.TestClass";

Class<?> loadTestClass1Type;

try (AeryCustomClassLoader aeryCustomClassLoader1 = new AeryCustomClassLoader(urls)) {

loadTestClass1Type = aeryCustomClassLoader1.loadClass(loadClassQualifiedName);

}

Class<?> loadTestClass2Type;

try (AeryCustomClassLoader aeryCustomClassLoader1 = new AeryCustomClassLoader(urls)) {

loadTestClass2Type = aeryCustomClassLoader1.loadClass(loadClassQualifiedName);

}

System.out.println();

System.out.println("loadTestClass1.equals(loadTestClass2) = " + loadTestClass1Type.equals(loadTestClass2Type));

Object loadTestClass1 = loadTestClass1Type.newInstance();

TestClass testClass; // 因為這裡出現TestClass此class, 因此將由Thread.currentThread().getContextClassLoader()載入

testClass = (TestClass) loadTestClass1;

testClass.print();

}

/** 列印預設的三個ClassLoader階層關係與搜尋的path */

private static void printInfo() {

Thread currentThread = Thread.currentThread();

ClassLoader contextClassLoader = currentThread.getContextClassLoader();

System.out.println();

printAllParentClassLoader(contextClassLoader, 1);

System.out.println();

printAllURLClassLoaderLoadURL((URLClassLoader) contextClassLoader);

printClassPathSystemProperty("sun.boot.class.path");

printClassPathSystemProperty("java.ext.dirs");

printClassPathSystemProperty("java.class.path");

}

private static void printClassPathSystemProperty(String propertyKey) {

String propertyPath = System.getProperty(propertyKey);

String[] paths = propertyPath.split(";");

System.out.println();

System.out.println(propertyKey);

for (String path : paths) {

System.out.println(" " + path);

}

public static void printAllParentClassLoader(ClassLoader classLoader, int offset) {

ClassLoader parentClassLoader = classLoader.getParent();

if (parentClassLoader != null) {

printAllParentClassLoader(parentClassLoader, offset + 1);

}

Class<? extends ClassLoader> classLoaderType = classLoader.getClass();

String msgFormat = String.format("%d %s", offset, classLoaderType.getName());

System.out.println(msgFormat + "\n" + fetchExtendsHierarchy(classLoaderType));

}

public static String fetchExtendsHierarchy(Class<?> classType) {

String title = " ";

Class<?> superClassType = classType.getSuperclass();

if (Object.class.equals(superClassType)) {

return title + classType.getName();

}

String rootsHierarchy = fetchExtendsHierarchy(superClassType);

return rootsHierarchy + "\n" + title + classType.getName();

}

public static void printAllURLClassLoaderLoadURL(URLClassLoader urlClassLoader) {

ClassLoader parentClassLoader = urlClassLoader.getParent();

if (parentClassLoader != null) {

printAllURLClassLoaderLoadURL((URLClassLoader) parentClassLoader);

}

System.out.println(urlClassLoader.getClass().getName());

URL[] loadUrls = urlClassLoader.getURLs();

for (URL url : loadUrls) {

System.out.println(" " + url);

}

AeryCustomClassLoader.java

package org.aery.practice.classloader;

import java.net.URL;

import java.net.URLClassLoader;

public class AeryCustomClassLoader extends URLClassLoader {

public AeryCustomClassLoader(URL[] urls) {

super(urls);

}

@Override

protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {

if ("org.aery.practice.classloader.TestClass".equals(name)) {

return super.findClass(name);

} else {

return super.loadClass(name, resolve);

}

TestClass.java

package org.aery.practice.classloader;

public class TestClass {

public void print() {

System.out.println(TestClass.class.getClassLoader());

}

/ 列印預設的三個ClassLoader階層關係與搜尋的path */**

printInfo();

2 sun.misc.Launcher$ExtClassLoader (繼承自URLClassLoder, 其parentClassLoader為BootStrapClassLoader)

java.lang.ClassLoader

java.security.SecureClassLoader

java.net.URLClassLoader

sun.misc.Launcher$ExtClassLoader

1 sun.misc.Launcher$AppClassLoader (繼承自URLClassLoder, 其parentClassLoader為ExtClassLoader)

java.lang.ClassLoader

java.security.SecureClassLoader

java.net.URLClassLoader

sun.misc.Launcher$AppClassLoader

sun.misc.Launcher$ExtClassLoader (列印ExtClassLoader給URLClassLoader的搜索路徑)

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/access-bridge-64.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/cldrdata.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/dnsns.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/jaccess.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/localedata.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/nashorn.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/sunec.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/sunjce_provider.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/sunmscapi.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/sunpkcs11.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/ext/zipfs.jar

sun.misc.Launcher$AppClassLoader (列印AppClassLoader給URLClassLoader的搜索路徑)

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/resources.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/rt.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/jsse.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/jce.jar

file:/C:/Program%20Files/ojdkbuild/java-1.8.0-openjdk-1.8.0.171-1/jre/lib/charsets.jar