Add configurable output stream to AnsiLog to avoid hardcoded System.out

[Copilot]

AnsiLog currently hardcodes System.out, which pollutes host stdout for embedded agents. This adds a configurable output stream while keeping default behavior.

• Configurable output: Introduced a volatile PrintStream out with setters/getters to override the log sink; defaults to System.out.
• Unified writes: All log printing and stack trace outputs now use the configurable stream.

// Redirect AnsiLog output
PrintStream prev = AnsiLog.out(new PrintStream("arthas.log"));
// ...
AnsiLog.out(prev); // restore if needed

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add a configurable out field to AnsiLog, similar to the existing LEVEL field</issue_title>
<issue_description>- [ ] 我已经在 issues 里搜索，没有重复的issue。

环境信息

When Arthas is embedded in other applications (e.g., OpenTelemetry Java Agent,
Spring Boot applications), AnsiLog outputs directly to System.out, which:

1. Pollutes the host application's stdout
2. Interferes with log collection systems
3. Cannot be redirected to a separate file

The official AgentBootstrap already uses a configurable PrintStream pattern,
but AnsiLog still hardcodes System.out.

Solution

Add a configurable out field to AnsiLog, similar to the existing LEVEL field:

• Default: System.out (backward compatible)
• API: AnsiLog.out(PrintStream) / AnsiLog.out()
• Thread-safe with volatile

期望的结果

diff --git a/common/src/main/java/com/taobao/arthas/common/AnsiLog.java b/common/src/main/java/com/taobao/arthas/common/AnsiLog.java
index xxx..yyy 100644
--- a/common/src/main/java/com/taobao/arthas/common/AnsiLog.java
+++ b/common/src/main/java/com/taobao/arthas/common/AnsiLog.java
@@ -1,5 +1,6 @@
 package com.taobao.arthas.common;
 
+import java.io.PrintStream;
 import java.util.logging.Level;
 import java.util.regex.Matcher;
 
@@ -20,6 +21,12 @@ public abstract class AnsiLog {
 
     static boolean enableColor;
 
+    /**
+     * Output stream for log messages, defaults to System.out.
+     * Can be configured via {@link #out(PrintStream)} for embedded scenarios.
+     */
+    private static volatile PrintStream out = System.out;
+
     public static java.util.logging.Level LEVEL = java.util.logging.Level.CONFIG;
 
     private static final String RESET = "\033[0m";
@@ -76,6 +83,28 @@ public abstract class AnsiLog {
         return enableColor;
     }
 
+    /**
+     * Set the output stream for log messages.
+     *
+     * @param printStream the output stream, null to use System.out
+     * @return the previous output stream
+     */
+    public static PrintStream out(PrintStream printStream) {
+        PrintStream old = out;
+        out = (printStream != null) ? printStream : System.out;
+        return old;
+    }
+
+    /**
+     * Get the current output stream.
+     *
+     * @return the current output stream
+     */
+    public static PrintStream out() {
+        return out;
+    }
+
     /**
      * set logger Level
      *
@@ -144,9 +173,9 @@ public abstract class AnsiLog {
     public static void trace(String msg) {
         if (canLog(Level.FINEST)) {
             if (enableColor) {
-                System.out.println(TRACE_COLOR_PREFIX + msg);
+                out.println(TRACE_COLOR_PREFIX + msg);
             } else {
-                System.out.println(TRACE_PREFIX + msg);
+                out.println(TRACE_PREFIX + msg);
             }
         }
     }
@@ -159,7 +188,7 @@ public abstract class AnsiLog {
 
     public static void trace(Throwable t) {
         if (canLog(Level.FINEST)) {
-            t.printStackTrace(System.out);
+            t.printStackTrace(out);
         }
     }
 
@@ -167,9 +196,9 @@ public abstract class AnsiLog {
     public static void debug(String msg) {
         if (canLog(Level.FINER)) {
             if (enableColor) {
-                System.out.println(DEBUG_COLOR_PREFIX + msg);
+                out.println(DEBUG_COLOR_PREFIX + msg);
             } else {
-                System.out.println(DEBUG_PREFIX + msg);
+                out.println(DEBUG_PREFIX + msg);
             }
         }
     }
@@ -182,7 +211,7 @@ public abstract class AnsiLog {
 
     public static void debug(Throwable t) {
         if (canLog(Level.FINER)) {
-            t.printStackTrace(System.out);
+            t.printStackTrace(out);
         }
     }
 
@@ -190,9 +219,9 @@ public abstract class AnsiLog {
     public static void info(String msg) {
         if (canLog(Level.CONFIG)) {
             if (enableColor) {
-                System.out.println(INFO_COLOR_PREFIX + msg);
+                out.println(INFO_COLOR_PREFIX + msg);
             } else {
-                System.out.println(INFO_PREFIX + msg);
+                out.println(INFO_PREFIX + msg);
             }
         }
     }
@@ -205,7 +234,7 @@ public abstract class AnsiLog {
 
     public static void info(Throwable t) {
         if (canLog(Level.CONFIG)) {
-            t.printStackTrace(System.out);
+            t.printStackTrace(out);
         }
     }
 
@@ -213,9 +242,9 @@ public abstract class AnsiLog {
     public static void warn(String msg) {
         if (canLog(Level.WARNING)) {
             if (enableColor) {
-                System.out.println(WARN_COLOR_PREFIX + msg);
+                out.println(WARN_COLOR_PREFIX + msg);
             } else {
-                System.out.println(WARN_PREFIX + msg);
+                out.println(WARN_PREFIX + msg);
             }
         }
     }
@@ -228,...

</details>



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- Fixes alibaba/arthas#3126

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