-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintGCTimeStamps
-Xloggc:/path/to/gc.log

spring.jvm.args=-XX:+PrintGCDetails -XX:+PrintGCDateStamps -Xloggc:/path/to/gc.log

-Dcom.sun.management.jmxremote
-Dcom.sun.management.jmxremote.port=9010
-Dcom.sun.management.jmxremote.authenticate=false
-Dcom.sun.management.jmxremote.ssl=false

SELECT s FROM java.util.HashMap s WHERE s.size > 1000

jmap -dump:format=b,file=heap.hprof <PID>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

management.endpoints.web.exposure.include=health,metrics,heapdump
management.endpoint.health.show-details=always

@Component
@Endpoint(id = "memory-status")
public class MemoryStatusEndpoint {
    
    @ReadOperation
    public Map<String, Object> memoryStatus() {
        Map<String, Object> status = new HashMap<>();
        Runtime runtime = Runtime.getRuntime();
        
        long totalMemory = runtime.totalMemory();
        long freeMemory = runtime.freeMemory();
        long maxMemory = runtime.maxMemory();
        long usedMemory = totalMemory - freeMemory;
        
        status.put("total", bytesToMB(totalMemory));
        status.put("free", bytesToMB(freeMemory));
        status.put("used", bytesToMB(usedMemory));
        status.put("max", bytesToMB(maxMemory));
        status.put("usagePercentage", usedMemory * 100.0 / maxMemory);
        
        return status;
    }
    
    private double bytesToMB(long bytes) {
        return bytes / (1024.0 * 1024.0);
    }
}

jstack <PID> > thread_dump.txt

jmap -histo:live <PID> | head -20

"http-nio-8080-exec-10" #43 daemon prio=5 os_prio=0 tid=0x00007f8b5c34e800 nid=0x7f82 waiting on condition [0x00007f8a3bf7c000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
        at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
        at org.apache.tomcat.util.threads.TaskQueue.take(TaskQueue.java:103)
        at org.apache.tomcat.util.threads.TaskQueue.take(TaskQueue.java:31)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1074)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1134)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Thread.java:748)

spring.datasource.hikari.register-mbeans=true

// 正確方式
try (Connection conn = dataSource.getConnection();
     PreparedStatement ps = conn.prepareStatement("SELECT * FROM users");
     ResultSet rs = ps.executeQuery()) {
    // 處理結果集
} catch (SQLException e) {
    logger.error("Database error", e);
}

// 錯誤方式 - 可能導致連接泄漏
Connection conn = null;
try {
    conn = dataSource.getConnection();
    // ...如果這里拋出異常，連接可能不會關閉
} finally {
    // 可能遺漏關閉或異常處理不當
}

lsof -p <PID> | wc -l

import org.openjdk.btrace.core.annotations.*;
import static org.openjdk.btrace.core.BTraceUtils.*;

@BTrace
public class MemoryLeakTracer {
    @OnMethod(
        clazz="com.example.service.CacheService",
        method="addToCache"
    )
    public static void traceAdd(@Self Object self, @ProbeClassName String pcn, @ProbeMethodName String pmn, Object key, Object value) {
        println("Adding to cache: " + str(key));
        println("Cache size: " + get(field(classOf("com.example.service.CacheService"), "cache", self), "size"));
    }
}

btrace <PID> MemoryLeakTracer.java

public class EventCollector {
    // 危險：無界靜態(tài)集合
    private static final List<Event> ALL_EVENTS = new ArrayList<>();
    
    public void recordEvent(Event event) {
        ALL_EVENTS.add(event);  // 不斷增長，從不清理
    }
}

public byte[] readFile(String path) throws IOException {
    FileInputStream fis = new FileInputStream(path);
    // 錯誤：未使用try-with-resources，可能導致文件句柄泄漏
    ByteArrayOutputStream buffer = new ByteArrayOutputStream();
    int data;
    while ((data = fis.read()) != -1) {
        buffer.write(data);
    }
    // fis未關閉!
    return buffer.toByteArray();
}

public class Outer {
    private final byte[] largeArray = new byte[10 * 1024 * 1024];
    
    public Runnable createTask() {
        // 非靜態(tài)內部類持有外部類引用，可能導致largeArray無法釋放
        return new Runnable() {
            @Override
            public void run() {
                System.out.println("Task running");
            }
        };
    }
}

@Service
public class ProductService {
    // 不限大小的緩存，沒有過期策略
    private final Map<String, Product> productCache = new HashMap<>();
    
    public Product getProduct(String id) {
        if (!productCache.containsKey(id)) {
            Product product = repository.findById(id);
            productCache.put(id, product);  // 持續(xù)增長
        }
        return productCache.get(id);
    }
}

// 無界隊列可能導致內存溢出
ExecutorService executor = new ThreadPoolExecutor(
    10, 10, 0L, TimeUnit.MILLISECONDS,
    new LinkedBlockingQueue<Runnable>()  // 無界隊列
);