Java如何输出对象个数？

<div class="article-content">
  <p>在Java编程中，统计对象的创建数量是调试和性能分析的重要技能，以下是<strong>四种实用方法</strong>及其实现代码：</p>
  <h3>方法1：静态计数器（基础版）</h3>
  <pre><code class="language-java">public class User {
    // 声明静态计数器
    private static int instanceCount = 0;
    public User() {
        // 每次创建对象时计数器+1
        instanceCount++;
    }
    // 获取对象数量的公共方法
    public static int getInstanceCount() {
        return instanceCount;
    }
    public static void main(String[] args) {
        new User();
        new User();
        System.out.println("当前对象数: " + User.getInstanceCount()); // 输出：2
    }
}</code></pre>
  <p><strong>特点：</strong>简单高效，适合单线程环境</p>
  <h3>方法2：线程安全计数器</h3>
  <pre><code class="language-java">public class Product {
    // 使用AtomicInteger保证线程安全
    private static AtomicInteger atomicCount = new AtomicInteger(0);
    public Product() {
        atomicCount.incrementAndGet();
    }
    // 工厂方法统计对象创建
    public static Product createInstance() {
        return new Product();
    }
    public static int getTotalCount() {
        return atomicCount.get();
    }
    public static void main(String[] args) {
        IntStream.range(0, 100).parallel().forEach(i -> Product.createInstance());
        System.out.println("多线程创建数量: " + Product.getTotalCount()); // 输出：100
    }
}</code></pre>
  <p><strong>优势：</strong>支持高并发场景，避免计数错误</p>
  <h3>方法3：引用队列监控（高级）</h3>
  <pre><code class="language-java">public class AdvancedCounter {
    private static final ReferenceQueue&lt;Object&gt; QUEUE = new ReferenceQueue&lt;&gt;();
    private static final Set&lt;WeakReference&lt;Object&gt;&gt; REFERENCES = new HashSet&lt;&gt;();
    private static int totalCreated = 0;
    public static &lt;T&gt; T track(T obj) {
        REFERENCES.add(new WeakReference&lt;&gt;(obj, QUEUE));
        totalCreated++;
        cleanQueue();
        return obj;
    }
    private static void cleanQueue() {
        while (QUEUE.poll() != null) {
            // 当对象被GC回收时自动移出
        }
    }
    // 获取存活对象数
    public static int getActiveCount() {
        cleanQueue();
        return (int) REFERENCES.stream().filter(ref -> ref.get() != null).count();
    }
    // 获取历史创建总数
    public static int getTotalCreated() {
        return totalCreated;
    }
}
// 使用示例
Object obj = AdvancedCounter.track(new Object());
System.gc();
System.out.println("存活对象数: " + AdvancedCounter.getActiveCount());</code></pre>
  <p><strong>应用场景：</strong>内存泄漏检测、对象生命周期监控</p>
  <h3>方法4：Java Agent字节码增强（生产环境）</h3>
  <p>通过<code>java.lang.instrument</code>在类加载时注入计数代码：</p>
  <pre><code class="language-java">public class ObjectCounterAgent {
    public static void premain(String args, Instrumentation inst) {
        inst.addTransformer((loader, className, classBeingRedefined, 
                            protectionDomain, classfileBuffer) -> {
            ClassReader reader = new ClassReader(classfileBuffer);
            ClassWriter writer = new ClassWriter(reader, ClassWriter.COMPUTE_MAXS);
            reader.accept(new ClassVisitor(Opcodes.ASM9, writer) {
                @Override
                public MethodVisitor visitMethod(int access, String name, 
                    String desc, String signature, String[] exceptions) {
                    MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
                    if ("&lt;init&gt;".equals(name)) {
                        return new MethodVisitor(Opcodes.ASM9, mv) {
                            @Override
                            public void visitInsn(int opcode) {
                                if (opcode == Opcodes.RETURN) {
                                    visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
                                    visitLdcInsn("对象已创建! 总数: " + ++Counter.total);
                                    visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
                                }
                                super.visitInsn(opcode);
                            }
                        };
                    }
                    return mv;
                }
            }, 0);
            return writer.toByteArray();
        });
    }
    static class Counter {
        static long total = 0;
    }
}</code></pre>
  <p><strong>启动参数：</strong><code>-javaagent:agent.jar</code></p>
  <h3>使用建议</h3>
  <div class="recommendation">
    <ul>
      <li><strong>简单项目</strong> → 选择静态计数器（方法1）</li>
      <li><strong>并发系统</strong> → 采用线程安全计数器（方法2）</li>
      <li><strong>性能调优</strong> → 使用引用队列监控（方法3）</li>
      <li><strong>生产环境</strong> → Java Agent方案（方法4）</li>
    </ul>
  </div>
  <h3>注意事项</h3>
  <div class="warning-block">
    <ol>
      <li><code>finalize()</code>方法已被废弃，不要依赖它做计数递减</li>
      <li>对象回收受JVM垃圾收集策略影响，存活计数存在延迟</li>
      <li>生产环境推荐结合JMX或Micrometer暴露计数指标</li>
    </ol>
  </div>
  <div class="conclusion">
    <p>根据需求选择合适方案：开发阶段用静态计数器快速验证，生产环境用Java Agent无侵入监控，关键业务建议同时记录对象创建堆栈信息，便于问题溯源。</p>
  </div>
</div>
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<footer>
  本文技术要点基于Oracle官方文档及《Effective Java》实践建议，引用ASM字节码操作库实现高级监控方案。
</footer>