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 * Copyright (c) 2016, 2020, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
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 *
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 */

package jdk.jfr;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

Meta annotation for defining new types of event metadata.
 In the following example, a transaction event is defined with two user-defined annotations, @Severity and @TransactionId. 
 @MetadataDefinition "Severity") "Value between 0 and 100 that indicates severity. 100 is most severe.") 
@Retention(RetentionPolicy.RUNTIME)
 * @Target({ElementType.TYPE
})
public @interface Severity {
    int value() default 50;
}
@MetadataDefinition

"Transaction Id")
@Relational
@Retention(RetentionPolicy.RUNTIME)
 * @Target({ElementType.FIELD
})
public @interface TransactionId {
}
@Severity(80)
 * @Label("Transaction Blocked")
 * class
TransactionBlocked extends Event {
    @TransactionId
    @Label("Transaction")
    long transactionId1;
    @TransactionId
    @Label("Transaction Blocker")
    long transactionId2;
}
} Adding @MetadataDefinition to the declaration of @Severity and @TransactionId ensures the information is saved by Flight Recorder.
Since:
9/**
 * Meta annotation for defining new types of event metadata.
 * <p>
 * In the following example, a transaction event is defined with two
 * user-defined annotations, {@code @Severity} and {@code @TransactionId}.
 *
 * <pre>{@literal
 * @MetadataDefinition
 * @Label("Severity")
 * @Description("Value between 0 and 100 that indicates severity. 100 is most severe.")
 * @Retention(RetentionPolicy.RUNTIME)
 * @Target({ElementType.TYPE})
 * public @interface Severity {
 *     int value() default 50;
 * }
 *
 * @MetadataDefinition
 * @Label("Transaction Id")
 * @Relational
 * @Retention(RetentionPolicy.RUNTIME)
 * @Target({ElementType.FIELD})
 * public @interface TransactionId {
 * }
 *
 * @Severity(80)
 * @Label("Transaction Blocked")
 * class TransactionBlocked extends Event {
 *     @TransactionId
 *     @Label("Transaction")
 *     long transactionId1;
 *
 *     @TransactionId
 *     @Label("Transaction Blocker")
 *     long transactionId2;
 * }
 * }</pre>
 *
 * Adding {@code @MetadataDefinition} to the declaration of {@code @Severity} and {@code @TransactionId}
 * ensures the information is saved by Flight Recorder.
 *
 * @since 9
 */
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE })
public @interface MetadataDefinition {
}