类加载过程是线程安全的吗？

答案是：是的。

ClassLoader.loadClass()源码：

Unless overridden, this method synchronizes on the result of  method during the entire class loading process.

除非是重写，那么这个整个加载过程都是同步的。

/**
     * Loads the class with the specified binary name.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * 

     *
     *   
 Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  
     *
     *   
 Invoke the {@link #loadClass(String) loadClass} method
     *   on the parent class loader.  If the parent is null the class
     *   loader built-in to the virtual machine is used, instead.  
     *
     *   
 Invoke the {@link #findClass(String)} method to find the
     *   class.  
     *
     * 
     *
     * 
 If the class was found using the above steps, and the
     * resolve flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting Class object.
     *
     * 
 Subclasses of ClassLoader are encouraged to override {@link
     * #findClass(String)}, rather than this method.  
     *
     * 
 Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock getClassLoadingLock} method
     * during the entire class loading process.
     *
     * @param  name
     *         The binary name of the class
     *
     * @param  resolve
     *         If true then resolve the class
     *
     * @return  The resulting Class object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    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;
        }
    }