murong00 opened a new issue #6561: [pulsar-client] NPE while call getLastMessageId URL: https://github.com/apache/pulsar/issues/6561 **Describe the bug** Meet NPE while call getLastMessageId as below: ``` [root@xxx apache-pulsar-2.6.0-SNAPSHOT]# bin/pulsar-admin topics last-message-id persistent://public/default/test java.lang.RuntimeException: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.apache.pulsar.client.impl.BatchMessageIdImpl["acker"]->org.apache.pulsar.client.impl.BatchMessageAcker["outstandingAcks"]) at org.apache.pulsar.admin.cli.CliCommand.print(CliCommand.java:182) at org.apache.pulsar.admin.cli.CmdTopics$GetLastMessageId.run(CmdTopics.java:773) at org.apache.pulsar.admin.cli.CmdBase.run(CmdBase.java:71) at org.apache.pulsar.admin.cli.PulsarAdminTool.run(PulsarAdminTool.java:221) at org.apache.pulsar.admin.cli.PulsarAdminTool.main(PulsarAdminTool.java:263) Caused by: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: org.apache.pulsar.client.impl.BatchMessageIdImpl["acker"]->org.apache.pulsar.client.impl.BatchMessageAcker["outstandingAcks"]) at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:394) at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:353) at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316) at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:730) at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166) at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727) at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722) at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:166) at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480) at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319) at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1433) at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1135) at com.fasterxml.jackson.databind.ObjectWriter.writeValueAsString(ObjectWriter.java:1005) at org.apache.pulsar.admin.cli.CliCommand.print(CliCommand.java:180) ... 4 more Caused by: java.lang.NullPointerException at org.apache.pulsar.client.impl.BatchMessageAcker.getOutstandingAcks(BatchMessageAcker.java:69) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:688) at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:722) ... 14 more ``` The reason is that `BatchMessageIdImpl` use a default `BatchMessageAcker` with null value. **Expected behavior** `getLastMessageId` should be successful like below: ``` [root@xxx apache-pulsar-2.6.0-SNAPSHOT]# bin/pulsar-admin topics last-message-id persistent://public/default/test { "ledgerId" : 5, "entryId" : 0, "partitionIndex" : -1, "batchIndex" : 9, "acker" : { "batchSize" : 0, "prevBatchCumulativelyAcked" : false, "outstandingAcks" : 0 }, "outstandingAcksInSameBatch" : 0, "batchSize" : 0 } ```
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