Android Annotation processors must be explicitly declared now..

使用 butterknife 这个库时，添加依赖：

implementation 'com.jakewharton:butterknife:5.1.1'

在gradle 2.1.2版本没问题，但是在gradle 3.0+ 的话就会报错：

Annotation processors must be explicitly declared now.  The following dependencies on the compile classpath are found to contain annotation processor.  Please add them to the annotationProcessor configuration.

添加了：

annotationProcessor "com.jakewharton:butterknife-compiler-5.1.1"

有提示找不到：

Could not find com.jakewharton:butterknife-compiler-5.1.1:.

这里提供一个更简便的方法：

 defaultConfig {
     
        minSdkVersion 21
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        vectorDrawables.useSupportLibrary = true
        javaCompileOptions {
            annotationProcessorOptions {
                includeCompileClasspath = true
            }
        }
    }

defaultConfig 这个闭包中添加：

  javaCompileOptions {
            annotationProcessorOptions {
                includeCompileClasspath = true
            }
        }

问题解决。。。。