From 8e6ce5ebe6dfa2fd2baab645c844d0e05ddd2873 Mon Sep 17 00:00:00 2001
From: Burak KALAYCI <kalayciburak1996@gmail.com>
Date: Tue, 4 Aug 2026 16:39:21 +0300
Subject: [PATCH] [#2514] Flush out/err PrintWriters after CommandLine.execute

print() does not trigger autoFlush on PrintWriter. Flush the root and
parsed command writers in a finally block so buffered output is not lost
when the JVM exits immediately after execute().
---
 src/main/java/picocli/CommandLine.java | 19 ++++++++++++
 src/test/java/picocli/ExecuteTest.java | 41 ++++++++++++++++++++++++++
 2 files changed, 60 insertions(+)

diff --git a/src/main/java/picocli/CommandLine.java b/src/main/java/picocli/CommandLine.java
index ab95179f1..a2cba9984 100644
--- a/src/main/java/picocli/CommandLine.java
+++ b/src/main/java/picocli/CommandLine.java
@@ -2187,6 +2187,25 @@ public int execute(String... args) {
             }
         } catch (Exception ex) {
             return handleUnhandled(ex, this, getCommandSpec().exitCodeOnExecutionException());
+        } finally {
+            // #2514 print() does not auto-flush; ensure out/err buffers are flushed after execute
+            flush(this);
+            if (parseResult[0] != null) {
+                for (CommandLine parsed : parseResult[0].asCommandLineList()) {
+                    flush(parsed);
+                }
+            }
+        }
+    }
+    private static void flush(CommandLine cmd) {
+        if (cmd == null) {
+            return;
+        }
+        if (cmd.out != null) {
+            cmd.out.flush();
+        }
+        if (cmd.err != null) {
+            cmd.err.flush();
         }
     }
     private static int handleUnhandled(Exception ex, CommandLine cmd, int defaultExitCode) {
diff --git a/src/test/java/picocli/ExecuteTest.java b/src/test/java/picocli/ExecuteTest.java
index 96c0f19b5..f1d7355dc 100644
--- a/src/test/java/picocli/ExecuteTest.java
+++ b/src/test/java/picocli/ExecuteTest.java
@@ -1626,5 +1626,46 @@ public int handleParseException(ParameterException ex, String[] args) throws Exc
         }
         assertEquals(expected, lines);
     }
+
+    @Command(name = "demo")
+    static class Issue2514Command implements Callable<Integer> {
+        @Spec CommandSpec spec;
+        private final boolean writeErr;
+
+        Issue2514Command(boolean writeErr) {
+            this.writeErr = writeErr;
+        }
+
+        public Integer call() {
+            if (writeErr) {
+                spec.commandLine().getErr().print("err-hello");
+            } else {
+                spec.commandLine().getOut().print("hello");
+            }
+            return 0;
+        }
+    }
+
+    @Test
+    public void testExecuteFlushesOutAfterPrintWithoutNewline() {
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        PrintWriter out = CommandLine.newPrintWriter(baos, getStdoutEncoding());
+
+        int exit = new CommandLine(new Issue2514Command(false)).setOut(out).execute();
+
+        assertEquals(ExitCode.OK, exit);
+        assertEquals("hello", baos.toString());
+    }
+
+    @Test
+    public void testExecuteFlushesErrAfterPrintWithoutNewline() {
+        ByteArrayOutputStream baos = new ByteArrayOutputStream();
+        PrintWriter err = CommandLine.newPrintWriter(baos, getStdoutEncoding());
+
+        int exit = new CommandLine(new Issue2514Command(true)).setErr(err).execute();
+
+        assertEquals(ExitCode.OK, exit);
+        assertEquals("err-hello", baos.toString());
+    }
 }