Reset cached AppendableByteArray before it is reused

core/spring-boot/src/main/java/org/springframework/boot/json/AppendableByteArray.java

@@ -129,6 +129,12 @@ static AppendableByteArray get(Charset charset) {
 			result = new AppendableByteArray(charset);
 			cache.set(new SoftReference<>(result));
 		}
+		else {
+			// The cached instance is reused, so it must be clean before it is
+			// handed out again. A previous use may have been abandoned part-way,
+			// for example when writing the value threw, leaving content behind.
+			result.reset();
+		}
 		return result;
 	}
 

core/spring-boot/src/test/java/org/springframework/boot/json/AppendableByteArrayTests.java

@@ -68,6 +68,18 @@ void writeUsingCache() throws IOException {
 		assertByteArray(StandardCharsets.UTF_8, AppendableByteArray::get, (appendable) -> appendable.append(string));
 	}
 
+	@Test
+	void getWhenPreviousUseWasAbandonedReturnsCleanInstance() throws IOException {
+		AppendableByteArray abandoned = AppendableByteArray.get(StandardCharsets.UTF_8);
+		abandoned.append("partial content");
+		// The instance is never converted to a byte array, as happens when writing
+		// the value throws part-way through.
+		AppendableByteArray reused = AppendableByteArray.get(StandardCharsets.UTF_8);
+		assertThat(reused).isSameAs(abandoned);
+		reused.append("clean");
+		assertThat(reused.toByteArray()).isEqualTo("clean".getBytes(StandardCharsets.UTF_8));
+	}
+
 	private void assertByteArray(Charset charset, ThrowingConsumer<Appendable> action) throws Exception {
 		assertByteArray(4, 4, charset, action);
 	}

core/spring-boot/src/test/java/org/springframework/boot/json/WritableJsonTests.java

@@ -66,6 +66,17 @@ void toByteArrayReturnsByteArray() {
 		assertThat(writable.toByteArray()).isEqualTo("{}".getBytes());
 	}
 
+	@Test
+	void toByteArrayWhenPreviousWriteFailedDoesNotIncludePartialContent() {
+		WritableJson failing = (out) -> {
+			out.append("{\"partial\":");
+			throw new IllegalStateException("bad");
+		};
+		assertThatExceptionOfType(IllegalStateException.class).isThrownBy(failing::toByteArray);
+		WritableJson healthy = (out) -> out.append("{\"ok\":true}");
+		assertThat(healthy.toByteArray()).isEqualTo("{\"ok\":true}".getBytes(StandardCharsets.UTF_8));
+	}
+
 	@Test
 	void toResourceWritesJson() throws Exception {
 		File file = new File(this.temp, "out.json");