ClassLoader類加載機制

       

                                             ExclassLoader                     

 

                                      AppClassLoader

 

 上圖類關係出自sun.misc.Launcher類，能夠看出ExclassLoader與 AppClassLoader都繼承與URLClassLoader，而非繼承Bootstrap ClassLoader

public class MyAppClassLoader {

    private static class Test {
        Test() {
            System.out.println("Hello World!");
        }
    }

    Launcher launcher = new Launcher();


    public static void main(String[] args) throws ClassNotFoundException {
        ClassLoader classLoader = MyAppClassLoader.class.getClassLoader();
        System.out.println(classLoader);
        System.out.println(classLoader.getParent());
        System.out.println(classLoader.getParent().getParent());
    }
}

"C:\Program Files\Java\jdk1.7.0_80\bin\java" "-javaagent:D:\idea\IntelliJ IDEA 2017.1.4\lib\idea_rt.jar=61577:D:\idea\IntelliJ IDEA 2017.1.4\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.7.0_80\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\jce.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\jfxrt.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\resources.jar;C:\Program Files\Java\jdk1.7.0_80\jre\lib\rt.jar;D:\ideaprojects\NewCoderTest\out\production\NewCoderTest" MyAppClassLoader
sun.misc.Launcher$AppClassLoader@7b7035c6
sun.misc.Launcher$ExtClassLoader@3da997a
null

Process finished with exit code 0

 ExclassLoader與 AppClassLoader都繼承與URLClassLoade，而URLClassLoade實現抽象類ClassLoader。

 

分析Launcher源碼

Launcher.class

public Launcher() {
        Launcher.ExtClassLoader var1;
        try {
            //先建立ExtClassLoader
            var1 = Launcher.ExtClassLoader.getExtClassLoader();
        } catch (IOException var10) {
            throw new InternalError("Could not create extension class loader");
        }

        try {
            //由ExtClassLoader再去建立它的子類加載器，再講將該類加載器置爲Launcher的默認類加載器
            this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
        } catch (IOException var9) {
            throw new InternalError("Could not create application class loader");
        }

        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);
        }

    }

ClassLoader.class

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;
        }
    }

 

追蹤到最後由該類進行調用 （看註釋能夠得出，做爲默認加載器被調用）
@CallerSensitive
public static ClassLoader getSystemClassLoader() {
    initSystemClassLoader();
    if (scl == null) {
        return null;
    }
    SecurityManager sm = System.getSecurityManager();
    if (sm != null) {
        checkClassLoaderPermission(scl, Reflection.getCallerClass());
    }
    return scl;
}

　　

看ClassLoader.class中的loadClass方法，探究類加載原理

    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        //getClassLoadingLock方法，經過類名獲取
        synchronized (getClassLoadingLock(name)) {
            // 檢查該類是否已經被加載過
            Class c = findLoadedClass(name);
            if (c == null) {
                //計時，輸出納秒級別時間
                long t0 = System.nanoTime();
                try {
                    //因而可知BootstrapClassLoader並非ExtClassLoader加載器的父加載器，僅僅只是在經過父加載優先行不通的狀況下，纔會去找BootstrapClassLoader
                    if (parent != null) {
                        //先查找父類加載器，false表示
                        c = parent.loadClass(name, false);
                    } else {
                        //查找BootstrapClass,找不到返回null
                        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();
                    //若是都沒找到經過本加載器加載
                    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;
        }
    }

 

private final ConcurrentHashMap<String, Object> parallelLockMap;
經過類名，若是paralleLockMap這個ConcurrentHashMap爲空，這返回ClassLoader,不然經過putIfAbsent源碼能夠看到，該方法若是一個HashEntry中可以找到該key對應value值則返回該value,
若是找不到，則返回null,則返回Object
protected Object getClassLoadingLock(String className) {
            Object lock = this;
            if (parallelLockMap != null) {
            Object newLock = new Object();
            //經過putIfAbsent源碼能夠看到，該方法若是一個HashEntry中可以找到該key對應value值則返回該value,若是不能找到，則返回null,下面會分析ConcurrentHashMap中的該源碼
            lock = parallelLockMap.putIfAbsent(className, newLock);
            if (lock == null) {
                lock = newLock;
            }
        }
        return lock;
    }

　　

protected final Class<?> findLoadedClass(String name) {
        if (!checkName(name))
            return null;
        return findLoadedClass0(name);
    }
 //native關鍵字說明其修飾的方法是一個原生態方法，方法對應的實現不是在當前文件，而是在用其餘語言（如C和C++）實現的文件中。
private native final Class findLoadedClass0(String name);

　　

總結：由上的代碼分析能夠分析出loadClass加載類的過程，以及對BootstrapClassLoader的誤解。