这篇文章聊一下Java中的ClassLoader。
前言 Link to heading
假如现在有一个class file叫做cn.nikeo.Square.class，位于**/home/lenox/IdeaProjects/kotlin-starter/**目录下，如下所示：
// IntelliJ API Decompiler stub source generated from a class file
// Implementation of methods is not available
package cn.nikeo;
public class Square {
private final int left;
private final int top;
private final int right;
private final int bottom;
public Square(int left, int top, int right, int bottom) { /* compiled code */ }
public int width() { /* compiled code */ }
public int height() { /* compiled code */ }
public int perimeter() { /* compiled code */ }
public int area() { /* compiled code */ }
}
我现在需要加载并使用它，有以下几种方式：
- 如果你使用Gradle，可以使用如下方式：
// build.gradle.kts
dependencies {
runtimeOnly(files("/home/lenox/IdeaProjects/kotlin-starter"))
}
- 在使用
java执行main class文件的时候，添加-classpath /home/lenox/IdeaProjects/kotlin-starter选项，如：
$ java -Dfile.encoding=UTF-8 \
-classpath /home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/rt.jar:/home/lenox/IdeaProjects/kotlin-starter \
Main
其实这两种方式都是将 /home/lenox/IdeaProjects/kotlin-starter 添加到 classpath，此时你就可以使用 /home/lenox/IdeaProjects/kotlin-starter 下的 class files 了。你可以通过 System.getProperty("java.class.path") 查看当前运行环境下所有的 classpath 。比如:
// println(System.getProperty("java.class.path").split(":").joinToString("\n"))
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/charsets.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/cldrdata.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/dnsns.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/jaccess.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/localedata.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/nashorn.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunec.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunjce_provider.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunpkcs11.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/zipfs.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jce.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jfr.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jsse.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/management-agent.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/resources.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/rt.jar
/home/lenox/IdeaProjects/kotlin-starter/build/classes/java/main
/home/lenox/IdeaProjects/kotlin-starter/build/classes/kotlin/main
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-jdk8/1.4.21/3ad7f99fb330947a12451ea16767d192d763600a/kotlin-stdlib-jdk8-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/com.squareup.okhttp3/okhttp/4.9.0/8e17601d3bdc8cf57902c154de021931d2c27c1/okhttp-4.9.0.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/io.reactivex.rxjava3/rxjava/3.0.8/b9ff4d4d216a088b337fde628a49d0f3233962e7/rxjava-3.0.8.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-jdk7/1.4.21/26b6082f9296911bdcb8e72a7cc68692c7025a03/kotlin-stdlib-jdk7-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib/1.4.21/4a668382d7c38688d3490afde93b6a113ed46698/kotlin-stdlib-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/com.squareup.okio/okio/2.8.0/49b64e09d81c0cc84b267edd0c2fd7df5a64c78c/okio-jvm-2.8.0.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.reactivestreams/reactive-streams/1.0.3/d9fb7a7926ffa635b3dcaa5049fb2bfa25b3e7d0/reactive-streams-1.0.3.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-common/1.4.21/7f48a062aa4b53215998780f7c245a4276828e1d/kotlin-stdlib-common-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains/annotations/13.0/919f0dfe192fb4e063e7dacadee7f8bb9a2672a9/annotations-13.0.jar
+/home/lenox/IdeaProjects/kotlin-starter
/home/lenox/.local/share/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.5981.155/lib/idea_rt.
现在你可以使用系统的ClassLoader去加载位于**/home/lenox/IdeaProjects/kotlin-starter/目录下的cn.nikeo.Square.class**了：
fun main() {
val clazz = ClassLoader.getSystemClassLoader().loadClass("cn.nikeo.Square")
val constructor = clazz.getConstructor(Int::class.java, Int::class.java, Int::class.java, Int::class.java)
val c = constructor.newInstance(10, 10, 100, 100)
println(clazz.getMethod("width").invoke(c)) // output: 90
println(clazz.getMethod("height").invoke(c)) // output: 90
println(clazz.getMethod("perimeter").invoke(c)) // output: 360
println(clazz.getMethod("area").invoke(c)) // output: 8100
}
现在介绍第三种方式-自定义ClassLoader
class LocalClassLoader(private val classPath: String) : ClassLoader() {
override fun findClass(name: String): Class<*> {
val className = name.replace(".", "/") + ".class"
val classFile = File(classPath, className)
require(classFile.exists() && classFile.isFile) {
"$classFile is not exist."
}
val b = loadClassData(classFile)
return defineClass(name, b, 0, b.size)
}
private fun loadClassData(classFile: File): ByteArray {
return classFile.inputStream().readBytes()
}
}
然后我们可以使用这个LocalClassLoader去加载位于**/home/lenox/IdeaProjects/kotlin-starter/目录下的cn.nikeo.Square.class**了：
fun main() {
val clazz = LocalClassLoader("/home/lenox/IdeaProjects/kotlin-starter").loadClass("cn.nikeo.Square")
val constructor = clazz.getConstructor(Int::class.java, Int::class.java, Int::class.java, Int::class.java)
val c = constructor.newInstance(10, 10, 100, 100)
println(clazz.getMethod("width").invoke(c)) // output: 90
println(clazz.getMethod("height").invoke(c)) // output: 90
println(clazz.getMethod("perimeter").invoke(c)) // output: 360
println(clazz.getMethod("area").invoke(c)) // output: 8100
}
ClassLoader Link to heading
ClassLoader是用来将Java类加载到JVM中，Java源代码经过Java编译器( javac 命令)编译成Java字节码文件后，该字节码文件被加载进JVM中解释执行，具体来说，该字节码文件由ClassLoader负责读取，然后将其转换成 java.lang.Class 类的一个实例（Java类），然后我们可以通过 getConstructor() 和 newInstance 创建出该类的一个对象。
分类 Link to heading
总的来说有四类ClassLoader
Bootstrap class loader Link to heading
这个ClassLoader主要用来加载Java的核心库，需要注意的是这个ClassLoader存在于 native，所以它并没有继承 java.lang.ClassLoader。
// java.lang.ClassLoader.java
/**
* Returns a class loaded by the bootstrap class loader;
* or return null if not found.
*/
private Class<?> findBootstrapClassOrNull(String name)
{
if (!checkName(name)) return null;
return findBootstrapClass(name);
}
// return null if not found
private native Class<?> findBootstrapClass(String name);
这个ClassLoader加载哪个路径下的资源呢，我们可以查看以下源码：
// sun.misc.Launcher.java
public class Launcher {
...
private static String bootClassPath =
System.getProperty("sun.boot.class.path");
...
private static class BootClassPathHolder {
static final URLClassPath bcp;
static {
URL[] urls;
if (bootClassPath != null) {
urls = AccessController.doPrivileged(
new PrivilegedAction<URL[]>() {
public URL[] run() {
File[] classPath = getClassPath(bootClassPath);
int len = classPath.length;
Set<File> seenDirs = new HashSet<File>();
for (int i = 0; i < len; i++) {
File curEntry = classPath[i];
// Negative test used to properly handle
// nonexistent jars on boot class path
if (!curEntry.isDirectory()) {
curEntry = curEntry.getParentFile();
}
if (curEntry != null && seenDirs.add(curEntry)) {
MetaIndex.registerDirectory(curEntry);
}
}
return pathToURLs(classPath);
}
}
);
} else {
urls = new URL[0];
}
bcp = new URLClassPath(urls, factory, null);
bcp.initLookupCache(null);
}
}
public static URLClassPath getBootstrapClassPath() {
return BootClassPathHolder.bcp;
}
}
由上面的源码可知，路径定义在 sun.boot.class.path 这个系统属性下，我们可以打印一下：
// println(System.getProperty("sun.boot.class.path").split(":").joinToString("\n"))
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/resources.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/rt.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/sunrsasign.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jsse.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jce.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/charsets.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jfr.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/classes
ExtClassLoader Link to heading
这个ClassLoader主要用来加载Java的拓展库，它是 java.lang.ClassLoader 的字类。
/*
* The class loader used for loading installed extensions.
*/
static class ExtClassLoader extends URLClassLoader {
...
private static File[] getExtDirs() {
String s = System.getProperty("java.ext.dirs");
File[] dirs;
if (s != null) {
StringTokenizer st =
new StringTokenizer(s, File.pathSeparator);
int count = st.countTokens();
dirs = new File[count];
for (int i = 0; i < count; i++) {
dirs[i] = new File(st.nextToken());
}
} else {
dirs = new File[0];
}
return dirs;
}
...
}
由上面的源码可知，路径定义在 java.ext.dirs 这个系统属性下，我们可以打印一下：
// println(System.getProperty("java.ext.dirs").split(":").joinToString("\n"))
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext
/usr/java/packages/lib/ext
AppClassLoader Link to heading
这个ClassLoader主要根据Java类路径(CLASSPATH)用来加载Java类，它是 java.lang.ClassLoader 的字类。
/**
* The class loader used for loading from java.class.path.
* runs in a restricted security context.
*/
static class AppClassLoader extends URLClassLoader {
...
public static ClassLoader getAppClassLoader(final ClassLoader extcl)
throws IOException
{
final String s = System.getProperty("java.class.path");
final File[] path = (s == null) ? new File[0] : getClassPath(s);
// Note: on bugid 4256530
// Prior implementations of this doPrivileged() block supplied
// a rather restrictive ACC via a call to the private method
// AppClassLoader.getContext(). This proved overly restrictive
// when loading classes. Specifically it prevent
// accessClassInPackage.sun.* grants from being honored.
//
return AccessController.doPrivileged(
new PrivilegedAction<AppClassLoader>() {
public AppClassLoader run() {
URL[] urls =
(s == null) ? new URL[0] : pathToURLs(path);
return new AppClassLoader(urls, extcl);
}
});
}
...
}
由上面的源码可知，路径定义在 java.class.path 这个系统属性下，我们可以打印一下：
// println(System.getProperty("java.class.path").split(":").joinToString("\n"))
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/charsets.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/cldrdata.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/dnsns.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/jaccess.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/localedata.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/nashorn.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunec.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunjce_provider.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/sunpkcs11.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/ext/zipfs.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jce.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jfr.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/jsse.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/management-agent.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/resources.jar
/home/lenox/.sdkman/candidates/java/8.0.275.open-adpt/jre/lib/rt.jar
/home/lenox/IdeaProjects/kotlin-starter/build/classes/java/main
/home/lenox/IdeaProjects/kotlin-starter/build/classes/kotlin/main
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-jdk8/1.4.21/3ad7f99fb330947a12451ea16767d192d763600a/kotlin-stdlib-jdk8-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/com.squareup.okhttp3/okhttp/4.9.0/8e17601d3bdc8cf57902c154de021931d2c27c1/okhttp-4.9.0.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/io.reactivex.rxjava3/rxjava/3.0.8/b9ff4d4d216a088b337fde628a49d0f3233962e7/rxjava-3.0.8.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-jdk7/1.4.21/26b6082f9296911bdcb8e72a7cc68692c7025a03/kotlin-stdlib-jdk7-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib/1.4.21/4a668382d7c38688d3490afde93b6a113ed46698/kotlin-stdlib-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/com.squareup.okio/okio/2.8.0/49b64e09d81c0cc84b267edd0c2fd7df5a64c78c/okio-jvm-2.8.0.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.reactivestreams/reactive-streams/1.0.3/d9fb7a7926ffa635b3dcaa5049fb2bfa25b3e7d0/reactive-streams-1.0.3.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains.kotlin/kotlin-stdlib-common/1.4.21/7f48a062aa4b53215998780f7c245a4276828e1d/kotlin-stdlib-common-1.4.21.jar
/home/lenox/.gradle/caches/modules-2/files-2.1/org.jetbrains/annotations/13.0/919f0dfe192fb4e063e7dacadee7f8bb9a2672a9/annotations-13.0.jar
/home/lenox/IdeaProjects/kotlin-starter
/home/lenox/.local/share/JetBrains/Toolbox/apps/IDEA-U/ch-0/203.5981.155/lib/idea_rt.jar
通过 ClassLoader.getSystemClassLoader() 获取到的就是 AppClassLoader:
// java.lang.ClassLoader.java
public abstract class ClassLoader {
public static ClassLoader getSystemClassLoader() {
initSystemClassLoader();
if (scl == null) {
return null;
}
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkClassLoaderPermission(scl, Reflection.getCallerClass());
}
return scl;
}
private static synchronized void initSystemClassLoader() {
if (!sclSet) {
if (scl != null)
throw new IllegalStateException("recursive invocation");
sun.misc.Launcher l = sun.misc.Launcher.getLauncher();
if (l != null) {
Throwable oops = null;
scl = l.getClassLoader();
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;
}
}
}
// sun.misc.Launcher.java
public class Launcher {
private static URLStreamHandlerFactory factory = new Factory();
private static Launcher launcher = new Launcher();
private static String bootClassPath =
System.getProperty("sun.boot.class.path");
public static Launcher getLauncher() {
return launcher;
}
private ClassLoader loader;
public Launcher() {
// Create the extension class loader
ClassLoader extcl;
try {
extcl = ExtClassLoader.getExtClassLoader();
} catch (IOException e) {
throw new InternalError(
"Could not create extension class loader", e);
}
// Now create the class loader to use to launch the application
try {
loader = AppClassLoader.getAppClassLoader(extcl);
} catch (IOException e) {
throw new InternalError(
"Could not create application class loader", e);
}
// Also set the context class loader for the primordial thread.
Thread.currentThread().setContextClassLoader(loader);
// Finally, install a security manager if requested
String s = System.getProperty("java.security.manager");
if (s != null) {
// init FileSystem machinery before SecurityManager installation
sun.nio.fs.DefaultFileSystemProvider.create();
SecurityManager sm = null;
if ("".equals(s) || "default".equals(s)) {
sm = new java.lang.SecurityManager();
} else {
try {
sm = (SecurityManager)loader.loadClass(s).newInstance();
} catch (IllegalAccessException e) {
} catch (InstantiationException e) {
} catch (ClassNotFoundException e) {
} catch (ClassCastException e) {
}
}
if (sm != null) {
System.setSecurityManager(sm);
} else {
throw new InternalError(
"Could not create SecurityManager: " + s);
}
}
}
/*
* Returns the class loader used to launch the main application.
*/
public ClassLoader getClassLoader() {
return loader;
}
}
Custom ClassLoader Link to heading
这个ClassLoader是用户根据实际需求自定义的ClassLoader, 如上面提到的 LocalClassLoader。
ClassLoader 的代理 Link to heading
按照 java.lang.ClassLoader类的构造器，可以说明每个ClassLoader都有一个 Parent ClassLoader，当通过 loadClass方法加载字节码文件的时候，会先代理给其 Parent ClassLoader，然后一层一层的传递。
// java.lang.ClassLoader.java
public abstract class ClassLoader {
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;
}
}
}
由上分析源码可知几个ClassLoader有以下关系：
BootstrapClassLoader(Native)
^
| parent（loadClass()）
ExtClassLoader(Java) --> (findClass()) --> (defineClass())
^
| parent（loadClass()）
AppClassLoader(Java) --> (findClass()) --> (defineClass())
当我们使用 ClassLoader.getSystemClassLoader() （AppClassLoader）去 loadClass() 的时候，会先调用 ExtClassLoader 的 loadClass()，由于 ExtClassLoader 没有 parent，所以会使用 BootstrapClassLoader去加载class file,如果在引导类路径中没有找到class file，则调用 ExtClassLoader 的 findClass()去寻找，找到了就调用 defineClass 去将class file转化为 java.lang.Class；如果ExtClassLoader 没有加载成功，则会向下继续调用 AppClassLoader 的 findClass()去寻找，找到了就调用 defineClass 去将class file转化为 java.lang.Class；加载是个从上到下的过程。
ClassLoader#findLoadedClass Link to heading
查找名称为name的已经被加载过的类，这样不用每次重新去加载：
try {
Class<?> aClass = ClassLoader.getSystemClassLoader().loadClass("cn.nikeo.Rect");
Class<?> bClass = ClassLoader.getSystemClassLoader().loadClass("cn.nikeo.Rect"); // findLoadedClass
System.out.println(aClass);
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
Class.forName() Link to heading
public static Class<?> forName(String name, boolean initialize, ClassLoader loader)
name：类的全限定名。initialize：是否初始化，如果初始化，则会调用该类的静态代码块。loader：类加载器，默认是 bootstrap class loader。
使用ClassLoader.getSystemClassLoader().loadClass() 不会调用该类的静态代码块。