My R studio version is 4.0. I installed xlsx package, but when I need to use it, I get an error:

library(xlsx)
Unable to find any JVMs matching version "(null)".
No Java runtime present, try --request to install.
Error: package or namespace load failed for ‘xlsx’:
 .onLoad failed in loadNamespace() for 'rJava', details:
  call: fun(libname, pkgname)
  error: JVM could not be found
In addition: Warning messages:
1: In system("/usr/libexec/java_home", intern = TRUE) :
  running command '/usr/libexec/java_home' had status 1
2: In fun(libname, pkgname) :
  Cannot find JVM library 'NA/lib/server/libjvm.dylib'
Install Java and/or check JAVA_HOME (if in doubt, do NOT set it, it will be detected)

So according to this note, I installed 'rJava' and want to load this package at first, but I still get an error:

library(rJava)
Unable to find any JVMs matching version "(null)".
No Java runtime present, try --request to install.
Error: package or namespace load failed for ‘rJava’:
 .onLoad failed in loadNamespace() for 'rJava', details:
  call: fun(libname, pkgname)
  error: JVM could not be found
In addition: Warning messages:
1: In system("/usr/libexec/java_home", intern = TRUE) :
  running command '/usr/libexec/java_home' had status 1
2: In fun(libname, pkgname) :
  Cannot find JVM library 'NA/lib/server/libjvm.dylib'
Install Java and/or check JAVA_HOME (if in doubt, do NOT set it, it will be detected)

Could someone give me some advice on how to load 'xlsx' and 'rJava' packages? Thank you.

The xlsx package depends on the rJava package, which requires a valid installation of the Java Runtime Environment (JRE). Before installing the rJava package, one must install the Java Runtime Environment (JRE). The process for installing the JRE varies by operating system, and Oracle has made this process more difficult due to changes they made in the licensing requirements for Java starting in 2019.

How do I tell whether the JRE is installed?

As noted in the comments on the question, one can verify the version of the Java runtime that is installed on a machine with the command line java -version. It is important to also confirm that the Java runtime is accessible from R / RStudio. We can do this by executing the system() function within RStudio, also as noted in the comments for the question.

> system("java -version")
java version "1.8.0_251"
Java(TM) SE Runtime Environment (build 1.8.0_251-b08)
Java HotSpot(TM) 64-Bit Server VM (build 25.251-b08, mixed mode)
[1] 0
> 

Reading Excel files into R

There are two distinct approaches to solving this problem. Here is a summary of instructions that I wrote on this topic for the Johns Hopkins Data Science Specialization Getting and Cleaning Data course, Common Problems: Java and xlsx package back in 2017.

Solution 1: Use an Excel Reader Package that Doesn't Require Java

PRO TIP: The easiest way to work around this problem is to use an R package that does not depend on Java, such as openxlsx or readxl.

For openxlsx, it's very easy.

  install.packages("openxlsx")
  library(openxlsx)
  # read the help file to identify the arguments needed to 
  # correctly read the file
  ?openxlsx
  theData <- read.xlsx(...)

The same process can be used for readxl.

  install.packages("readxl")
  library(readxl)
  # read the help file to identify the arguments needed to 
  # correctly read the file 
  ?readxl
  theData <- read_excel(...)

Note that there is a complementary package, writexl, that enables people to write Excel files.

Solution 2: Install Java and Required R Packages

That said, for people who want to use the xlsx package to work with Excel files, there are workable solutions for Windows, Mac OSX, and Ubuntu Linux.

SOLUTION (Windows): Download and install the latest version of the Java Runtime Environment from Oracle. Note that if you are running the 64-bit version of R, you need to install the 64-bit version of the Java Runtime.

SOLUTION (Mac OSX): As of newer releases of Mac OSX, this has become more complicated. A specific set of commands needs to be followed after installing the Java Development Kit on the computer. These are documented on the rJava Issue 86 github page. I have included a screenshot of this solution for people to reference directly.

SOLUTION (Ubuntu): Use the Ubuntu Advanced Packaging Tool to install Java, then reconfigure Java in R.

  sudo apt-get install openjdk-8-jdk # openjdk-9-jdk has some installation issues
  sudo R CMD javareconf

Then in R / RStudio install the xlsx package.

  install.packages("xlsx")

32-bit vs. 64-bit Java in Windows

Another common problem people may encounter is an incompatibility between the version of the Java Runtime Environment that is installed on their computer and the version of R, either 32-bit or 64-bit.

For example, if one has installed the 64-bit version of R but has the 32-bit version of Java Runtime Environment installed, R will not have visibility to the Java Runtime Environment, generating the same "Java not installed error" as noted above.

SOLUTION: This problem can be resolved by either installing the 64-bit version of Java Runtime for Windows, or by changing the RStudio configuration to use the 32-bit version of R.