使用mailR包发送邮件时遭遇Java错误，寻求解决方法

Let's break down what's happening here and walk through the fixes:

Why this error pops up #

The javax/activation/DataHandler class belongs to the Java Activation Framework (JAF), which was included by default in Java 8 and older versions. Starting with Java 9, Oracle removed this module from the core JDK—and since mailR hasn't been updated in years, it still depends on this missing class. That's why your code creates the email object without issues, but fails when you try to send it: the critical class isn't available at runtime.

Solution 1: Switch to Java 8 (most reliable fix) #

MailR was built to work seamlessly with Java 8, so rolling back to this version will eliminate the error entirely:

1. Check your current Java version by running this in your terminal/command prompt:

   java -version
   

2. Download and install Java 8 from an official distribution (Oracle or OpenJDK both work).
3. Tell R to use Java 8 by setting the JAVA_HOME environment variable before loading rJava or mailR:

   # Windows example (update the path to match your Java 8 installation)
   Sys.setenv(JAVA_HOME = "C:/Program Files/Java/jdk1.8.0_391")
   
   # macOS/Linux example
   Sys.setenv(JAVA_HOME = "/Library/Java/JavaVirtualMachines/jdk1.8.0_391.jdk/Contents/Home")
   

4. Restart R completely, then reload mailR and run your code again.

Solution 2: Add the missing JAF jar to your classpath (keep using Java 9+) #

If you don't want to switch Java versions, you can manually add the required library to R's Java classpath:

1. Download the activation-1.1.1.jar file (the latest version compatible with mailR) from a trusted source like Maven Central.
2. Save the jar to a stable location on your computer (e.g., ~/R/libs/activation-1.1.1.jar for macOS/Linux, C:\R\libs\activation-1.1.1.jar for Windows).
3. Before running your mailR code, load rJava and add the jar to the classpath:

   library(rJava)
   .jaddClassPath("~/R/libs/activation-1.1.1.jar") # Update to your actual file path
   library(mailR)
   
   # Run your original email setup and send code
   email <- send.mail(from = sender, to = recipients, subject = "Test", body = "This is a test", 
                      smtp = list(host.name = "smtp.mail.com", port = 587, user.name = sender, passwd = pw), 
                      authenticate = T, send = F)
   email$send()
   

Alternative: Use a modern email package #

If you're open to switching tools, consider emayili—it's actively maintained, supports modern Java versions, and has a simpler API:

install.packages("emayili")
library(emayili)

# Build your email
email <- envelope() %>%
  from(sender) %>%
  to(recipients) %>%
  subject("Test") %>%
  body("This is a test")

# Send via SMTP
smtp_server <- server(host = "smtp.mail.com", port = 587, username = sender, password = pw)
smtp_server(email, send = TRUE)

内容的提问来源于stack exchange，提问作者Tim Hargreaves