关于Jackson的ObjectMapper单例模式的问题

首先ObjectMapper如果是线程安全的才能使用单例模式，测试表时它是线程安全的。
接下来进行单例模式和多例模式的性能测试，每个模式都测试两轮（单线程，多线程)。

单例模式

import cn.ec.entity.AgentEntity;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.concurrent.CountDownLatch;

/**
 * 测试Jackson的ObjectMapper性能
 */
@RunWith(SpringRunner.class)
@SpringBootTest
public class TestJackSonIsSingle {

    private static final ObjectMapper objectMapper = new ObjectMapper();

    /**
     * 单例模式
     *
     * @return
     */
    private ObjectMapper getObjectMapper() {
        return objectMapper;
    }

    /**
     * 单线程模式
     *
     * @throws JsonProcessingException
     */
    @Test
    public void testSingleThread() throws JsonProcessingException {
        long start = System.currentTimeMillis();
        for (int i = 0; i < 100; i++) {
            getObjectMapper().writeValueAsString(
                    new AgentEntity().setId(i).setName(String.valueOf(i))
            );
        }
        System.out.println("最终用时:" + (System.currentTimeMillis() - start) + "ms");//30,14,14,平均值:19.33
    }

    /**
     * 多线程模式
     *
     * @throws InterruptedException
     */
    @Test
    public void testMulThread() throws InterruptedException {
        final int count = 100;
        CountDownLatch countDownLatch = new CountDownLatch(count);
        long start = System.currentTimeMillis();
        for (int i = 0; i < count; i++) {
            int finalI = i;
            new Thread(() -> {
                try {
                    getObjectMapper().writeValueAsString(
                            new AgentEntity().setId(finalI).setName(String.valueOf(finalI))
                    );
                } catch (JsonProcessingException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            }).start();
        }
        countDownLatch.await();
        System.out.println("最终用时:" + (System.currentTimeMillis() - start) + "ms");//79,153,67,平均值:99.66
    }
}

测试结果

• 单线程19.33毫秒
• 多线程99.66毫秒

多例模式

import cn.ec.entity.AgentEntity;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.concurrent.CountDownLatch;

/**
 * 测试Jackson的ObjectMapper性能
 */
@RunWith(SpringRunner.class)
@SpringBootTest
public class TestJackSonIsSingle {

    /**
     * 多例模式
     *
     * @return
     */
    private ObjectMapper getObjectMapper() {
        return new ObjectMapper();
    }

    /**
     * 单线程模式
     *
     * @throws JsonProcessingException
     */
    @Test
    public void testSingleThread() throws JsonProcessingException {
        long start = System.currentTimeMillis();
        for (int i = 0; i < 100; i++) {
            getObjectMapper().writeValueAsString(
                    new AgentEntity().setId(i).setName(String.valueOf(i))
            );
        }
        System.out.println("最终用时:" + (System.currentTimeMillis() - start) + "ms");//153,125,135,平均值:137.66
    }

    /**
     * 多线程模式
     *
     * @throws InterruptedException
     */
    @Test
    public void testMulThread() throws InterruptedException {
        final int count = 100;
        CountDownLatch countDownLatch = new CountDownLatch(count);
        long start = System.currentTimeMillis();
        for (int i = 0; i < count; i++) {
            int finalI = i;
            new Thread(() -> {
                try {
                    getObjectMapper().writeValueAsString(
                            new AgentEntity().setId(finalI).setName(String.valueOf(finalI))
                    );
                } catch (JsonProcessingException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            }).start();
        }
        countDownLatch.await();
        System.out.println("最终用时:" + (System.currentTimeMillis() - start) + "ms");//225,173,161,平均值:186.33
    }
}

测试结果

• 单线程137.66毫秒
• 多线程186.33毫秒

结论

模式	单线程耗时(ms)	多线程耗时(ms)
单例模式	19.33	99.66
多例模式	137.66	186.33

测试证明：不管是多线程还是单线程，Jackson的ObjectMapper在单例模式下性能优于多例模式。