SpringBoot 启动源码解析系列二：判断是否 Web 程序

2018-01-02

SpringApplication 类中含有如下方法：

private boolean deduceWebEnvironment() {
    String[] var1 = WEB_ENVIRONMENT_CLASSES;
    int var2 = var1.length;

    for(int var3 = 0; var3 < var2; ++var3) {
        String className = var1[var3];
        if (!ClassUtils.isPresent(className, (ClassLoader)null)) {
            return false;
        }
    }

    return true;
}

其中 WEB_ENVIRONMENT_CLASSES 是 SpringApplication 持有的静态属性：

1 2	private static final String[] WEB_ENVIRONMENT_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};

SpringApplication 判断是否是 Web 程序的逻辑非常简单，即判断当前类路径下是否包含 Servlet 和 ConfigurableWebApplicationContext 类：

public static boolean isPresent(String className, ClassLoader classLoader) {
    try {
        forName(className, classLoader);
        return true;
    } catch (Throwable var3) {
        return false;
    }
}

public static Class<?> forName(String name, ClassLoader classLoader) throws ClassNotFoundException, LinkageError {
    Assert.notNull(name, "Name must not be null");
    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 {
            ClassLoader clToUse = classLoader;
            if (classLoader == null) {
                clToUse = getDefaultClassLoader();
            }

            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;
            }
        }
    }
}

以 WEB_ENVIRONMENT_CLASSES 参数来说，ClassUtils.isPresent(className, (ClassLoader)null) 方法将始终返回true，即默认就是 Web 程序