fix the bug about thread pool (#1623)[backend]

backend/src/main/java/heartbeat/config/ThreadPoolConfig.java

@@ -4,15 +4,18 @@
 import org.springframework.context.annotation.Configuration;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
 
+import java.util.concurrent.ThreadPoolExecutor;
+
 @Configuration
 public class ThreadPoolConfig {
 
 	@Bean("customTaskExecutor")
 	public ThreadPoolTaskExecutor taskExecutor() {
 		ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
-		executor.setCorePoolSize(20);
-		executor.setMaxPoolSize(100);
-		executor.setQueueCapacity(500);
+		executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
+		executor.setCorePoolSize(40);
+		executor.setMaxPoolSize(200);
+		executor.setQueueCapacity(1000);
 		executor.setKeepAliveSeconds(60);
 		executor.setThreadNamePrefix("Heartbeat-");
 		executor.initialize();