https://blog.csdn.net/txba6868/article/details/80732475

springboot在啟動的時候需要檢測當前專案是否是一個web專案，檢測方式為判斷classpath中是否有以下final變數中定義的兩個引數：

  private static final String[] WEB_ENVIRONMENT_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};

原始碼：

程式碼來源：org.springframework.boot.SpringApplication.initialize(Oeject[] sources)
 
this.webEnvironment = this.deduceWebEnvironment();  //檢測當前專案是否為web專案

程式碼來源：org.springframework.boot.SpringApplication.deduceWebEnvironment()
 
private boolean deduceWebEnvironment() {
        String[] var1 = WEB_ENVIRONMENT_CLASSES;
        int var2 = var1.length;
        //從全域性變數中取出標識web專案的引數，遍歷
        for(int var3 = 0; var3 < var2; ++var3) {
            String className = var1[var3];
            if (!ClassUtils.isPresent(className, (ClassLoader)null)) { 
                return false;
            }
        }
        return true;
    }

程式碼來源：org.springframework.util.ClassUtils.isPresent(String className,ClassLoader classLoc)
 
public static boolean isPresent(String className, ClassLoader classLoader) {
        try {
            forName(className, classLoader);
            return true;
        } catch (Throwable var3) {
            return false;
        }
    }

程式碼來源：org.springframework.util.ClassUtils.forName(String name,ClassLoader classLoader)
 
public static Class<?> forName(String name, ClassLoader classLoader) throws ClassNotFoundException, LinkageError {
        Assert.notNull(name, "Name must not be null");
        //首先檢查記憶體中當前的className是否存在
        Class<?> clazz = resolvePrimitiveClassName(name); 
        if (clazz == null) {
            clazz = (Class)commonClassCache.get(name);
        }
       
        if (clazz != null) {
            return clazz;
        } else {
            Class elementClass;
            String elementName;
            if (name.endsWith("[]")) {
                elementName = name.substring(0, name.length() - "[]".length());
                elementClass = forName(elementName, classLoader);
                return Array.newInstance(elementClass, 0).getClass();
            } else if (name.startsWith("[L") && name.endsWith(";")) {
                elementName = name.substring("[L".length(), name.length() - 1);
                elementClass = forName(elementName, classLoader);
                return Array.newInstance(elementClass, 0).getClass();
            } else if (name.startsWith("[")) {
                elementName = name.substring("[".length());
                elementClass = forName(elementName, classLoader);
                return Array.newInstance(elementClass, 0).getClass();
            } else {
                //記憶體中不存在時將呼叫系統的classLoader從系統指定路徑進行載入
                ClassLoader clToUse = classLoader;
                if (classLoader == null) {
                    clToUse = getDefaultClassLoader();
                }
                //載入執行
                try {
                    return clToUse != null ? clToUse.loadClass(name) : Class.forName(name);
                } catch (ClassNotFoundException var9) {
                    int lastDotIndex = name.lastIndexOf(46);
                    if (lastDotIndex != -1) {
                        String innerClassName = name.substring(0, lastDotIndex) + '$' + name.substring(lastDotIndex + 1);
 
                        try {
                            return clToUse != null ? clToUse.loadClass(innerClassName) : Class.forName(innerClassName);
                        } catch (ClassNotFoundException var8) {
                            ;
                        }
                    }
 
                    throw var9;
                }
            }
        }
    }

上述程式碼提到了類載入（classLoader）。具體的類載入機制請看這裡。

關於類載入的介紹，轉自： https://blog.csdn.net/briblue/article/details/54973413。