Index: src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java
===================================================================
--- src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java	(revision 1341589)
+++ src/main/org/apache/tools/ant/taskdefs/compilers/DefaultCompilerAdapter.java	(working copy)
@@ -417,14 +417,14 @@
     /**
      * Do the compile with the specified arguments.
      * @param args - arguments to pass to process on command line
-     * @param firstFileName - index of the first source file in args,
+     * @param separableArgIndex - index of the first source file in args,
      * if the index is negative, no temporary file will ever be
      * created, but this may hit the command line length limit on your
      * system.
      * @return the exit code of the compilation
      */
-    protected int executeExternalCompile(String[] args, int firstFileName) {
-        return executeExternalCompile(args, firstFileName, true);
+    protected int executeExternalCompile(String[] args, int separableArgIndex) {
+        return executeExternalCompile(args, separableArgIndex, true);
     }
 
     /**
@@ -434,10 +434,14 @@
      * project's base directory.</p>
      *
      * @param args - arguments to pass to process on command line
-     * @param firstFileName - index of the first source file in args,
-     * if the index is negative, no temporary file will ever be
-     * created, but this may hit the command line length limit on your
-     * system.
+     * @param separableArgIndex - index of the first argument which could be
+     * placed in a separate file, if the total command line size exceeds some
+     * platform-dependent threshold (typically 4Kb). This and subsequent
+     * arguments will be replaced with {@code @that-file-location}. May be
+     * {@code -1} to suppress this feature entirely. Use {@code 0} to replace
+     * all arguments, which is suitable for all modern compilers. Some very old
+     * compilers require this to be the index of the first actual source file,
+     * in case compiler options cannot be included.
      * @param quoteFiles - if set to true, filenames containing
      * spaces will be quoted when they appear in the external file.
      * This is necessary when running JDK 1.4's javac and probably
@@ -446,7 +450,7 @@
      *
      * @since Ant 1.6
      */
-    protected int executeExternalCompile(String[] args, int firstFileName,
+    protected int executeExternalCompile(String[] args, int separableArgIndex,
                                          boolean quoteFiles) {
         String[] commandArray = null;
         File tmpFile = null;
@@ -460,13 +464,13 @@
              * file if the total length of the command line exceeds this limit.
              */
             if (Commandline.toString(args).length() > COMMAND_LINE_LIMIT
-                && firstFileName >= 0) {
+                && separableArgIndex >= 0) {
                 BufferedWriter out = null;
                 try {
                     tmpFile = FILE_UTILS.createTempFile(
                         "files", "", getJavac().getTempdir(), true, true);
                     out = new BufferedWriter(new FileWriter(tmpFile));
-                    for (int i = firstFileName; i < args.length; i++) {
+                    for (int i = separableArgIndex; i < args.length; i++) {
                         if (quoteFiles && args[i].indexOf(" ") > -1) {
                             args[i] = args[i].replace(File.separatorChar, '/');
                             out.write("\"" + args[i] + "\"");
@@ -476,9 +480,9 @@
                         out.newLine();
                     }
                     out.flush();
-                    commandArray = new String[firstFileName + 1];
-                    System.arraycopy(args, 0, commandArray, 0, firstFileName);
-                    commandArray[firstFileName] = "@" + tmpFile;
+                    commandArray = new String[separableArgIndex + 1];
+                    System.arraycopy(args, 0, commandArray, 0, separableArgIndex);
+                    commandArray[separableArgIndex] = "@" + tmpFile;
                 } catch (IOException e) {
                     throw new BuildException("Error creating temporary file",
                                              e, location);
Index: src/main/org/apache/tools/ant/taskdefs/compilers/AptExternalCompilerAdapter.java
===================================================================
--- src/main/org/apache/tools/ant/taskdefs/compilers/AptExternalCompilerAdapter.java	(revision 1341589)
+++ src/main/org/apache/tools/ant/taskdefs/compilers/AptExternalCompilerAdapter.java	(working copy)
@@ -56,13 +56,13 @@
         cmd.setExecutable(apt.getAptExecutable());
         setupModernJavacCommandlineSwitches(cmd);
         AptCompilerAdapter.setAptCommandlineSwitches(apt, cmd);
-        int firstFileName = cmd.size();
+        int separableArgIndex = Project.toBoolean(project.getProperty("pre-50781")) ? cmd.size() : 1;
         //add the files
         logAndAddFilesToCompile(cmd);
 
         //run
         return 0 == executeExternalCompile(cmd.getCommandline(),
-                firstFileName,
+                separableArgIndex,
                 true);
 
     }
Index: src/main/org/apache/tools/ant/taskdefs/compilers/JavacExternal.java
===================================================================
--- src/main/org/apache/tools/ant/taskdefs/compilers/JavacExternal.java	(revision 1341589)
+++ src/main/org/apache/tools/ant/taskdefs/compilers/JavacExternal.java	(working copy)
@@ -50,16 +50,16 @@
         } else {
             setupJavacCommandlineSwitches(cmd, true);
         }
-        int firstFileName = assumeJava11() ? -1 : cmd.size();
+        int separableArgIndex = assumeJava11() ? -1 : Project.toBoolean(project.getProperty("pre-50781")) ? cmd.size() : 1;
         logAndAddFilesToCompile(cmd);
         //On VMS platform, we need to create a special java options file
         //containing the arguments and classpath for the javac command.
         //The special file is supported by the "-V" switch on the VMS JVM.
         if (Os.isFamily("openvms")) {
-            return execOnVMS(cmd, firstFileName);
+            return execOnVMS(cmd, separableArgIndex);
         }
         return
-                executeExternalCompile(cmd.getCommandline(), firstFileName,
+                executeExternalCompile(cmd.getCommandline(), separableArgIndex,
                         true)
                 == 0;
     }
@@ -67,10 +67,10 @@
     /**
      * helper method to execute our command on VMS.
      * @param cmd
-     * @param firstFileName
+     * @param separableArgIndex
      * @return
      */
-    private boolean execOnVMS(Commandline cmd, int firstFileName) {
+    private boolean execOnVMS(Commandline cmd, int separableArgIndex) {
         File vmsFile = null;
         try {
             vmsFile = JavaEnvUtils.createVmsJavaOptionFile(cmd.getArguments());
@@ -78,7 +78,7 @@
                                     "-V",
                                     vmsFile.getPath()};
             return 0 == executeExternalCompile(commandLine,
-                            firstFileName,
+                            separableArgIndex,
                             true);
 
         } catch (IOException e) {
Index: WHATSNEW
===================================================================
--- WHATSNEW	(revision 1341589)
+++ WHATSNEW	(working copy)
@@ -7,6 +7,12 @@
 Fixed bugs:
 -----------
 
+ * <javac> could create excessively long command lines when e.g. long classpaths
+   were given. Now fixed for contemporary compilers; if using very old compilers
+   which do not accept options in @argumentfiles, use -Dpre-50781=true or the
+   corresponding <property>.
+   Bugzilla Report 50781.
+
  * External XML catalog resolver failed to use project basedir when given an
    unmentioned relative path like the internal resolver does.
    Bugzilla Report 52754.