Fix #501 - Do not run javac when it is just a stub asking you to install Java on Mac OSX

ale_linters/java/javac.vim

@@ -6,6 +6,27 @@ let s:classpath_sep = has('unix') ? ':' : ';'
 let g:ale_java_javac_options = get(g:, 'ale_java_javac_options', '')
 let g:ale_java_javac_classpath = get(g:, 'ale_java_javac_classpath', '')
 
+" Detect if the javac command just shows an annoying popup for Mac OSX.
+if has('macunix')
+    function s:GetIsJavacAnAppStoreStub() abort
+        let l:path = resolve(systemlist('which javac')[0])
+
+        for l:line in readfile(l:path)
+            " This string is present inside the executable for the popup.
+            if l:line =~? 'No Java runtime present'
+                return 1
+            endif
+        endfor
+
+        return 0
+    endfunction
+
+    let s:is_javac_an_app_store_stub = s:GetIsJavacAnAppStoreStub()
+    delfunction s:GetIsJavacAnAppStoreStub
+else
+    let s:is_javac_an_app_store_stub = 0
+endif
+
 function! ale_linters#java#javac#GetImportPaths(buffer) abort
     let l:pom_path = ale#path#FindNearestFile(a:buffer, 'pom.xml')
 
@@ -32,6 +53,11 @@ function! s:BuildClassPathOption(buffer, import_paths) abort
 endfunction
 
 function! ale_linters#java#javac#GetCommand(buffer, import_paths) abort
+    " If running the command will just show a popup, then don't run it.
+    if s:is_javac_an_app_store_stub
+        return ''
+    endif
+
     let l:cp_option = s:BuildClassPathOption(a:buffer, a:import_paths)
     let l:sp_option = ''