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package com.example.springboot;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HelloController {

	@Autowired
	private HelloService helloService;

	@GetMapping("/")
	public String index() {
		return "Greetings from Spring Boot!";
	}

	@GetMapping("/allocate")
	public void allocate(){
		helloService.allocate4GB();
	}

	@GetMapping("/getSize")
	public String getListSize(){
		return String.valueOf(helloService.getMemoryListSize());
	}

	@GetMapping("/getBytesSize")
	public String getBytesSize(){
		return String.valueOf(helloService.getBytesSize());
	}

	@GetMapping("/getListToString")
	public String getListToString(){
		return helloService.getListToString();
	}

}

package com.example.springboot;

import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

@Service
public class HelloService{

    private List<byte[]> memoryList = new ArrayList<>();

    public void allocate4GB() {
        try {
            long totalMemoryInBytes = 4L * 1024 * 1024 * 1024; // 4GB

            int chunkSize = 1024 * 1024 * 1024; // 1GB
            int numChunks = (int) (totalMemoryInBytes / chunkSize);

            for (int i = 0; i < numChunks; i++) {
                byte[] data = new byte[chunkSize];
                Arrays.fill(data, (byte) 'A'); // Fill array with character 'A'
                memoryList.add(data);
            }

            System.out.println("Allocated 4GB of memory using chunks.");
        } catch (OutOfMemoryError e) {
            System.err.println("Out of memory!");
        }
    }

    public long getMemoryListSize(){
        return memoryList.size();
    }

    public long getBytesSize(){
        if (getMemoryListSize() <= 0){
            return 0;
        }
        return memoryList.get(0).length;
    }

    public String getListToString(){
        StringBuffer sb = new StringBuffer();
        if (getMemoryListSize() <= 0){
            return "empty!";
        }
        for (byte[] bytes : memoryList){
            sb.append(bytes[0]);
        }
        return sb.toString();
    }
}

tovi@toviserver:~/Projects/CRaC/example-spring-boot$ ./mvnw package

tovi@toviserver:~/Projects/CRaC/example-spring-boot$ $JAVA_HOME/bin/java -XX:CRaCCheckpointTo=checkpoint/cr -Xms1g -Xmx6g -jar target/example-spring-boot-0.0.1-SNAPSHOT.jar

# It can be found that the memory used at this time is 4.2GB
tovi@toviserver:~/Projects/CRaC/example-spring-boot/checkpoint$ free -h
               total        used        free      shared  buff/cache   available
memory：       15Gi       4.2Gi       8.5Gi       395Mi       2.9Gi        10Gi
swap：      2.0Gi       0.0Ki       2.0Gi

# And you can find that no large memory has been allocated at this time
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getBytesSize
0
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getSize
0
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getListToString
empty!

# This situation requires writing 4GB of memory (all character arrays of 'A')
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/allocate

# The console will print Allocated 4GB of memory using chunks.

# It can be found that the memory used at this time is 8.2GB. Compared with the large memory that was not written before, 4GB of memory has been added, indicating that the large memory has been written.
tovi@toviserver:~/Projects/CRaC/example-spring-boot/checkpoint$ free -h
               total        used        free      shared  buff/cache   available
memory：       15Gi       8.2Gi       4.4Gi       395Mi       2.9Gi       6.7Gi
swap：      2.0Gi       0.0Ki       2.0Gi

# It can be found that the size of the bytes array at this time is 1024 * 1024 * 1024 = 1GB, and the starting character is 65, which is 'A'
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getBytesSize
1073741824
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getSize
4
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getListToString
65656565

tovi@toviserver:~/Projects/CRaC/example-spring-boot$ jcmd target/example-spring-boot-0.0.1-SNAPSHOT.jar JDK.checkpoint
143664:
CR: Checkpoint ...

# After completion, you can find that there is a checkpoint dump file in the cr directory and the size is 4.3GB, which is consistent with the 4GB large memory written.
tovi@toviserver:~/Projects/CRaC/example-spring-boot/checkpoint$ du -sh cr/
4.3G    cr/

# At this point the service has been paused and the memory has been released
tovi@toviserver:~/Projects/CRaC/example-spring-boot/checkpoint$ free -h
               total        used        free      shared  buff/cache   available
memory：       15Gi       4.0Gi       4.3Gi       395Mi       7.2Gi        10Gi
swap：      2.0Gi       0.0Ki       2.0Gi

tovi@toviserver:~/Projects/CRaC/example-spring-boot$ $JAVA_HOME/bin/java -XX:CRaCRestoreFrom=checkpoint/cr
2024-01-09T16:00:08.278+08:00  INFO 143664 --- [Attach Listener] o.s.c.support.DefaultLifecycleProcessor  : Restarting Spring-managed lifecycle beans after JVM restore
2024-01-09T16:00:08.293+08:00  INFO 143664 --- [Attach Listener] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port 8080 (http) with context path ''
2024-01-09T16:00:08.295+08:00  INFO 143664 --- [Attach Listener] o.s.c.support.DefaultLifecycleProcessor  : Spring-managed lifecycle restart completed (restored JVM running for 74 ms)

# At this time, the memory usage has not reached 8.4GB? why is that?
tovi@toviserver:~/Projects/CRaC/example-spring-boot/checkpoint$ free -h
               total        used        free      shared  buff/cache   available
memory：       15Gi       4.1Gi       4.3Gi       395Mi       7.2Gi        10Gi
swap：      2.0Gi       0.0Ki       2.0Gi

# At this time, according to the provided interface, it can be seen that the large array written is still there, and it is found that the service is still available, indicating that the restore is successful.
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getBytesSize
1073741824
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getSize
4
tovi@toviserver:~/Projects/CRaC/example-spring-boot$ curl http://localhost:8080/getListToString
65656565

java.lang.IllegalStateException: Cannot load configuration class: com.example.springboot.Application
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.enhanceConfigurationClasses(ConfigurationClassPostProcessor.java:414) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanFactory(ConfigurationClassPostProcessor.java:254) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:282) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:126) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:694) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:532) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:780) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:412) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:333) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1277) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1265) ~[spring-boot-2.0.5.RELEASE.jar!/:2.0.5.RELEASE]
	at com.example.springboot.Application.main(Application.java:13) ~[classes!/:0.0.1-SNAPSHOT]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:48) ~[spring-boot-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT]
	at org.springframework.boot.loader.Launcher.launch(Launcher.java:87) ~[spring-boot-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT]
	at org.springframework.boot.loader.Launcher.launch(Launcher.java:50) ~[spring-boot-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT]
	at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:51) ~[spring-boot-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT]
Caused by: java.lang.ExceptionInInitializerError: null
	at org.springframework.context.annotation.ConfigurationClassEnhancer.newEnhancer(ConfigurationClassEnhancer.java:122) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassEnhancer.enhance(ConfigurationClassEnhancer.java:110) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.enhanceConfigurationClasses(ConfigurationClassPostProcessor.java:403) ~[spring-context-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	... 20 common frames omitted
Caused by: org.springframework.cglib.core.CodeGenerationException: java.lang.reflect.InaccessibleObjectException-->Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @198e2867
	at org.springframework.cglib.core.ReflectUtils.defineClass(ReflectUtils.java:464) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator.generate(AbstractClassGenerator.java:336) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:93) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:91) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.internal.LoadingCache$2.call(LoadingCache.java:54) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[na:na]
	at org.springframework.cglib.core.internal.LoadingCache.createEntry(LoadingCache.java:61) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.internal.LoadingCache.get(LoadingCache.java:34) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData.get(AbstractClassGenerator.java:116) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:291) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.KeyFactory$Generator.create(KeyFactory.java:221) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.KeyFactory.create(KeyFactory.java:174) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.KeyFactory.create(KeyFactory.java:153) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.proxy.Enhancer.<clinit>(Enhancer.java:73) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	... 23 common frames omitted
Caused by: java.lang.reflect.InaccessibleObjectException: Unable to make protected final java.lang.Class java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain) throws java.lang.ClassFormatError accessible: module java.base does not "opens java.lang" to unnamed module @198e2867
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:354) ~[na:na]
	at java.base/java.lang.reflect.AccessibleObject.checkCanSetAccessible(AccessibleObject.java:297) ~[na:na]
	at java.base/java.lang.reflect.Method.checkCanSetAccessible(Method.java:199) ~[na:na]
	at java.base/java.lang.reflect.Method.setAccessible(Method.java:193) ~[na:na]
	at org.springframework.cglib.core.ReflectUtils$1.run(ReflectUtils.java:61) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at java.base/java.security.AccessController.doPrivileged(AccessController.java:569) ~[na:na]
	at org.springframework.cglib.core.ReflectUtils.<clinit>(ReflectUtils.java:52) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.KeyFactory$Generator.generateClass(KeyFactory.java:243) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	at org.springframework.cglib.core.AbstractClassGenerator.generate(AbstractClassGenerator.java:329) ~[spring-core-5.0.9.RELEASE.jar!/:5.0.9.RELEASE]
	... 35 common frames omitted