Sync OutputCapture from Spring Boot

See spring-projects/spring-boot#46685

Signed-off-by: Johnny Lim <[email protected]>

Author: izeye

micrometer-core/src/test/java/io/micrometer/core/testsupport/system/OutputCapture.java

@@ -48,11 +48,15 @@ class OutputCapture implements CapturedOutput {
 
     private final Deque<SystemCapture> systemCaptures = new ArrayDeque<>();
 
-    private final AtomicReference<String> out = new AtomicReference<>(null);
+    private final AtomicReference<Object> out = new AtomicReference<>();
 
-    private final AtomicReference<String> err = new AtomicReference<>(null);
+    private final AtomicReference<Object> err = new AtomicReference<>();
 
-    private final AtomicReference<String> all = new AtomicReference<>(null);
+    private final AtomicReference<Object> all = new AtomicReference<>();
+
+    OutputCapture() {
+        clearExisting();
+    }
 
     /**
      * Push a new system capture session onto the stack.
@@ -128,20 +132,21 @@ void reset() {
     }
 
     void clearExisting() {
-        this.out.set(null);
-        this.err.set(null);
-        this.all.set(null);
+        this.out.set(new NoOutput());
+        this.err.set(new NoOutput());
+        this.all.set(new NoOutput());
     }
 
-    private String get(AtomicReference<String> existing, Predicate<Type> filter) {
+    private String get(AtomicReference<Object> existing, Predicate<Type> filter) {
         Assert.state(!this.systemCaptures.isEmpty(),
                 "No system captures found. Please check your output capture registration.");
-        String result = existing.get();
-        if (result == null) {
-            result = build(filter);
-            existing.compareAndSet(null, result);
+        Object existingOutput = existing.get();
+        if (existingOutput instanceof String) {
+            return (String) existingOutput;
         }
-        return result;
+        String builtOutput = build(filter);
+        existing.compareAndSet(existingOutput, builtOutput);
+        return builtOutput;
     }
 
     String build(Predicate<Type> filter) {
@@ -306,4 +311,8 @@ enum Type {
 
     }
 
+    static class NoOutput {
+
+    }
+
 }