Android编译App的Java版本问题

前言

最近遇到一个问题，在我开发的工程里边引入了一个新的模块，然后在编译App的时候出现了编译失败，显示的错误日志如下：

AGPBI: {"kind":"error","text":"Invoke-customs are only supported starting with Android O (--min-api 26)","sources":[{}],"tool":"D8"}
AGPBI: {"kind":"error","text":"Default interface methods are only supported starting with Android N (--min-api 24): void android.arch.lifecycle.DefaultLifecycleObserver.onCreate(android.arch.lifecycle.LifecycleOwner)","sources":[{}],"tool":"D8"}

定睛一看，咦，这莫非是要我把最低支持提升到Android O（API 26），那可坏了，业务不允许啊。于是上网搜了一把，原来是因为Java编译的版本问题，只需要把Java的编译版本改到Java 1.8即可。

compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_8
    targetCompatibility JavaVersion.VERSION_1_8
}

为什么？

问题很顺利解决，但是为什么会出现这个问题呢？
于是我顺着android.arch.lifecycle.LifecycleOwner这个类进去看

/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.arch.lifecycle;

import android.support.annotation.NonNull;

/**
 * A class that has an Android lifecycle. These events can be used by custom components to
 * handle lifecycle changes without implementing any code inside the Activity or the Fragment.
 *
 * @see Lifecycle
 */
@SuppressWarnings({"WeakerAccess", "unused"})
public interface LifecycleOwner {
    /**
     * Returns the Lifecycle of the provider.
     *
     * @return The lifecycle of the provider.
     */
    @NonNull
    Lifecycle getLifecycle();
}

这个类确实是使用了Java 1.8 的新特性，在interface中直接定义了默认方法。
这个类来自于 android.arch.lifecycle:common-1.1.1.jar是被间接依赖进来的。